Chapter 18:  The Endocrine System

 

18–1  Intercellular Communication

 In a few specialized cases, cellular activities are coordinated by the exchange of ions and molecules from one cell to the next across gap junctions. This direct communication occurs between two cells of the same type, and the cells must be in extensive physical contact. The two cells communicate so closely that they function as a single entity. For example, gap junctions (1) coordinate ciliary movement among epithelial cells, (2) coordinate the contractions of cardiac muscle cells, and (3) facilitate the propagation of action potentials from one neuron to the next at electrical synapses.

Direct communication is highly specialized and relatively rare. Most of the communication between cells involves the release and receipt of chemical messages. The use of chemical messengers to transfer information from cell to cell within a single tissue is called paracrine communication . The chemicals involved are called paracrine factors, also known as cytokines, or local hormones .

 

Some paracrine factors, including several of the prostaglandins and related chemicals, have primary effects in their tissues of origin and secondary effects in other tissues and organs. When secondary effects occur, the paracrine factors are also acting as hormones. Hormones are chemical messengers that are released in one tissue and transported in the bloodstream to reach specific cells in other tissues.

 

 

Each hormone has target cells , specific cells that respond to its presence. These cells possess the receptors needed to bind and "read" the hormonal message.  The other hormones are treated like junk mail and ignored, because the cell lacks the receptors to read the messages they contain. The use of hormones to coordinate cellular activities in tissues in distant portions of the body is called endocrine communication .

 

A hormone may

 

stimulate the synthesis of an enzyme or a structural protein not already present in the cytoplasm by activating appropriate genes in the cell nucleus;

increase or decrease the rate of synthesis of a particular enzyme or other protein by changing the rate of transcription or translation; or

turn an existing enzyme "on" or "off" by changing its shape or structure.

 

The nervous system also relies primarily on chemical communication, but it does not use the bloodstream to deliver messages. Instead, neurons release a neurotransmitter at a synapse very close to the target cells that bear the appropriate receptors.  This form of synaptic communication is ideal for crisis management: If you are in danger of being hit by a speeding bus, the nervous system can coordinate and direct your leap to safety.  the differences between the nervous and endocrine systems seem relatively clear.

18–2  An Overview of the Endocrine System

 The endocrine system includes all the endocrine cells and tissues of the body which produce hormones or paracrine factors that have effects beyond their tissues of origin.

 

 

Hormone Structure
Hormones can be divided into three groups on the basis of their chemical structure: (1) amino acid derivatives , (2) peptide hormones , and (3) lipid derivatives .

 

Amino Acid Derivatives are relatively small molecules that are structurally related to amino acids, the building blocks of proteins. This group of hormones, sometimes known as the biogenic amines , includes epinephrine, norepinephrine, dopamine, the thyroid hormonboth E and NE are secreted by the adrenal medullae during sympathetic activation.

 

 Peptide Hormones are chains of amino acids. In general, peptide hormones are produced as prohormones–inactive molecules that are converted to active hormones either before or after they are secreted.
Peptide hormones can be divided into two groups. One large and diverse group includes hormones that range from short polypeptide chains, such as antidiuretic hormone (ADH) and oxytocin, to small proteins, such as growth hormone and prolactin .

 

The second group of peptide hormones consists of glycoproteins. These proteins are more than 200 amino acids long and have carbohydrate side chains. The glycoproteins include thyroid–stimulating hormone (TSH) , luteinizing hormone (LH) , and follicle–stimulating hormone (FSH) from the anterior lobe of the pituitary gland, as well as several hormones produced in other organs.

 

Lipid Derivatives
There are two classes of lipid derivatives : (1) steroid hormones , derived from cholesterol, and (2) eicosanoids , derived from arachidonic  acid , a 20–carbon fatty acid.
Steroid Hormones Steroid hormones are lipids structurally similar to cholesterol. Steroid hormones are released by male and female reproductive organs ( androgens by the testes, estrogens and progestins by the ovaries), the adrenal glands ( corticosteroids ), and the kidneys ( calcitriol ).

 

Eicosanoids are small molecules with a five–carbon ring at one end. These compounds are important paracrine factors that coordinate cellular activities and affect enzymatic processes (such as blood clotting) that occur in extracellular fluids. Some of the eicosanoids also have secondary roles as hormones. Examples of important eicosanoids include the following:

Secretion and Distribution of Hormones
Hormone release typically occurs where capillaries are abundant, and the hormones quickly enter the bloodstream for distribution throughout the body. A freely circulating hormone remains functional for less than one hour, and sometimes for as little as two minutes. It is inactivated when (1) it diffuses out of the bloodstream and binds to receptors in target tissues, (2) it is absorbed and broken down by cells of the liver or kidneys, or (3) it is broken down by enzymes in the plasma or interstitial fluids.

Thyroid hormones and steroid hormones remain in circulation much longer, because when these hormones enter the bloodstream, almost all of them become attached to special transport proteins. Thus, the bloodstream contains a substantial reserve (several weeks' supply) of these hormones at any time.

 

Mechanisms of Hormone Action

 

Hormone receptors are located either (1) on the cell membrane or (2) inside the cell.

 

Hormones and the Cell Membrane
The receptors for catecholamines (E, NE, and dopamine), peptide hormones, and eicosanoids are in the cell membranes of their respective target cells.

 

Because catecholamines and peptide hormones are not lipid soluble, they are unable to penetrate a cell membrane. Instead, these hormones bind to receptor proteins at the outer surface of the cell membrane. Eicosanoids, which are lipid soluble, diffuse across the membrane to reach receptor proteins on the inner surface of the membrane.

 

 

 

Hormones and Intracellular Receptors
Steroid hormones diffuse across the lipid part of the cell membrane and bind to receptors in the cytoplasm or nucleus. The hormone–receptor complexes then activate or deactivate specific genes. By this mechanism, steroid hormones can alter the rate of DNA transcription in the nucleus, and thus change the pattern of protein synthesis.

 

 

 



Control of Endocrine Activity

 

Endocrine reflexes are the functional counterparts of neural reflexes. They can be triggered by (1) humoral stimuli (changes in the composition of the extracellular fluid), (2) hormonal stimuli (the arrival or removal of a specific hormone), or (3) neural stimuli (the arrival of neurotransmitter at neuroglandular junctions). In most cases, endocrine reflexes are controlled by negative feedback mechanisms: A stimulus triggers the production of a hormone whose direct or indirect effects reduce the intensity of the stimulus.
Endocrine cells in a simple endocrine reflex involve only one hormone. The endocrine cells involved respond directly to changes in the composition of the extracellular fluid. The secreted hormone adjusts the activities of target cells and restores homeostasis. Simple endocrine reflexes control hormone secretion by the heart, pancreas, parathyroid gland, and digestive tract.

More complex endocrine reflexes involve one or more intermediary steps and two or more hormones. The hypothalamus, the highest level of endocrine control, integrates the activities of the nervous and endocrine systems.

 

  1. The hypothalamus secretes regulatory hormones , special hormones that control endocrine cells in the pituitary gland. (By convention, the use of hormone in the name indicates that the substance's identity is known; if the identity is not known, the term factor is used instead.) The hypothalamic regulatory hormones control the secretory activities of endocrine cells in the anterior lobe of the pituitary gland. The hormones released by the anterior lobe, in turn, control the activities of endocrine cells in the thyroid, adrenal cortex, and reproductive organs.
  2. The hypothalamus itself acts as an endocrine organ. Hypothalamic neurons synthesize hormones, transport them along axons within the infundibulum, and release them into the circulation at the posterior lobe of the pituitary gland.
  3. The hypothalamus contains autonomic centers that exert direct neural control over the endocrine cells of the adrenal medullae. When the sympathetic division is activated, the adrenal medullae release hormones into the bloodstream.

 

 

18–3  The Pituitary Gland  hangs inferior to the hypothalamus, connected by the slender, funnel–shaped structure called the infundibulum. The base of the infundibulum lies between the optic chiasm and the mamillary bodies. The pituitary gland is cradled by the sella turcica and held in position by the diaphragma sellae , a dural sheet that encircles the infundibulum. The diaphragma sellae locks the pituitary gland in position and isolates it from the cranial cavity.

 

The pituitary gland can be divided into posterior and anterior lobes on the basis of function and developmental anatomy. Nine important peptide hormones are released by the pituitary gland–seven by the anterior lobe and two by the posterior lobe.

 

The Anterior Lobeof the pituitary gland, or adenohypophysis, contains a variety of endocrine cells. The anterior lobe can be subdivided into three regions: (1) the pars distalis, which is the largest and most anterior portion of the pituitary gland; (2) an extension called the pars tuberalis , which wraps around the adjacent portion of the infundibulum; and (3) the slender pars intermedia , which forms a narrow band bordering the posterior lobe

 

 

Hypothalamic Control of the Anterior Lobe
Two classes of hypothalamic regulatory hormones exist: (1) releasing hormones and (2) inhibiting hormones. A releasing hormone ( RH ) stimulates the synthesis and secretion of one or more hormones at the anterior lobe, whereas an inhibiting hormone ( IH ) prevents the synthesis and secretion of hormones from the anterior lobe. An endocrine cell in the anterior lobe may be controlled by releasing hormones, inhibiting hormones, or some combination of the two. The regulatory hormones released at the hypothalamus are transported directly to the anterior lobe by the hypophyseal portal system.

 

 

Hormones of the Anterior Lobe

 

Thyroid–stimulating hormone Thyroid–stimulating hormone ( TSH ), or thyrotropin , targets the thyroid gland and triggers the release of thyroid hormones. TSH is released in response to thyrotropin–releasing hormone ( TRH ) from the hypothalamus. As circulating concentrations of thyroid hormones rise, the rates of TRH and TSH production decline.

Adrenocorticotropic hormone Adrenocorticotropic hormone ( ACTH ), also known as corticotropin , stimulates the release of steroid hormones by the adrenal cortex , the outer portion of the adrenal gland. ACTH specifically targets cells that produce gluco–corticoids, hormones that affect glucose metabolism. ACTH release occurs under the stimulation of corticotropin–releasing hormone ( CRH ) from the hypothalamus. As glucocorticoid levels increase, the rates of CRH release and ACTH release decline.

 

The Gonadotropins Follicle–stimulating hormone and luteinizing hormone are called gonadotropins, because they regulate the activities of the gonads . (These organs–the testes and ovaries in males and females, respectively–produce reproductive cells as well as hormones.) The production of gonadotropins occurs under stimulation by gonadotropin–releasing hormone (GnRH) from the hypothalamus. An abnormally low production of gonadotropins produces hypogonadism . Children with this condition will not undergo sexual maturation, and adults with hypogonadism cannot produce functional sperm or oocytes.

Follicle–stimulating hormone ( FSH ), or follitropin , promotes follicle development in females and, in combination with luteinizing hormone, stimulates the secretion of estrogens ) by ovarian cells. Estradiol is the most important estrogen. In males, FSH stimulates sustentacular cells , specialized cells in the tubules where sperm differentiate. In response, the sustentacular cells promote the physical maturation of developing sperm. FSH production is inhibited by inhibin , a peptide hormone released by cells in the testes and ovaries.

Luteinizing hormone ( LH ), or lutropin , induces ovulation , the production of reproductive cells in females. It also promotes the secretion, by the ovaries, of estrogens and the progestins (such as progesterone ), which prepare the body for possible pregnancy. In males, this gonadotropin is sometimes called interstitial cell–stimulating hormone (ICSH) , because it stimulates the production of sex hormones by the interstitial cells of the testes. These sex hormones are called androgens, the most important of which is testosterone . LH production, like FSH production, is stimulated by GnRH from the hypothalamus. GnRH production is inhibited by estrogens, progestins, and androgens.

Prolactin ( PRL ), or mammotropin , works with other hormones to stimulate mammary gland development. In pregnancy and during the nursing period that follows delivery, PRL also stimulates milk production by the mammary glands. The functions of PRL in males are poorly understood, but evidence indicates that PRL helps regulate androgen production by making interstitial cells more sensitive to LH.
Prolactin production is inhibited by prolactin–inhibiting hormone ( PIH )–the neurotransmitter dopamine. The hypothalamus also secretes a prolactin–releasing hormone, but the identity of this prolactin–releasing factor ( PRF ) is a mystery. Circulating PRL stimulates PIH release and inhibits the secretion of PRF .

Although PRL exerts the dominant effect on the glandular cells, normal development of the mammary glands is regulated by the interaction of several hormones. Prolactin, estrogens, progesterone, glucocorticoids, pancreatic hormones, and hormones produced by the placenta cooperate in preparing the mammary glands for secretion, and milk ejection occurs only in response to oxytocin release at the posterior lobe of the pituitary gland. Growth Hormone

 

Growth hormone ( GH ), or somatotropin, stimulates cell growth and replication by accelerating the rate of protein synthesis.

 

The direct actions of GH are:

 

The production of GH is regulated by growth hormone– releasing hormone ( GH–RH , or somatocrinin ) and growth hormone–inhibiting hormone ( GH–IH , or somatostatin ) from the hypothalamus. Somatomedins stimulate GH–IH and inhibit GH–RH.

Melanocyte–Stimulating Hormone The pars intermedia may secrete two forms of melanocyte–stimulating hormone (MSH) , or melanotropin . As the name indicates, MSH stimulates the melanocytes of the skin, increasing their production of melanin, a brown, black, or yellow–brown pigment. The release of MSH is inhibited by dopamine.

Melanocyte stimulating hormone is important in the control of skin pigmentation in fishes, amphibians, reptiles, and many mammals other than primates. The pars intermedia in adult humans is virtually nonfunctional, and the circulating blood usually does not contain MSH. However, MSH is secreted by the human pars intermedia (1) during fetal development, (2) in very young children, (3) in pregnant women, and (4) in some diseases. The functional significance of MSH secretion under these circumstances is not known. The administration of a synthetic form of MSH causes the skin to darken, so MSH has been suggested as a means of obtaining a "sunless tan."

 

Growth Hormone Abnormalities

 

Growth hormone stimulates muscular and skeletal development. If it is administered before the epiphyseal cartilages have closed, it will cause an increase in height, weight, and muscle mass. In extreme cases, gigantism can result. In acromegaly, an excessive amount of GH is released after puberty, when most of the epiphyseal cartilages have already closed. Cartilages and small bones respond to the hormone, however, resulting in abnormal growth of the hands, feet, lower jaw, skull, and clavicle.

 

 

Acromegaly results from the overproduction of growth hormone after the epiphyseal cartilages have closed. Bone shapes change, and cartilaginous areas of the skeleton enlarge. Notice the broad facial features and the enlarged lower jaw.

 

 

 

 

 

 

Children who are unable to produce adequate concentrations of GH have pituitary growth failure. The steady growth and maturation that typically precede and accompany puberty do not occur in these individuals, who have short stature, slow epiphyseal growth, and larger–than–normal adipose tissue reserves.

 

Normal growth patterns can be restored by the administration of GH. Before the advent of gene splicing and recombinant DNA techniques, GH had to be carefully extracted and purified from the pituitary glands of cadavers at considerable expense and risk of infectious disease. Now genetically manipulated bacteria are used to produce human GH in commercial quantities.

 

The current availability of purified human growth hormone has led to its use under medically questionable circumstances. For example, it is now being praised as an "antiaging" miracle cure. Although GH supplements do slow or even reverse the losses in bone and muscle mass that accompany aging, little is known about adverse side effects that may accompany long–term use of the hormone in mature adults.

 

GH is also being sought by some parents of short but otherwise healthy children. These parents view short stature as a handicap that merits treatment by a physician. Whether we are considering GH treatment of adults or children, it is important to remember that GH and the somatomedins affect many different tissues and have widespread metabolic impacts. For example, children exposed to GH may grow faster, but their body fat content declines drastically and sexual maturation is delayed. The decline is associated with metabolic changes in many organs. The range and significance of these metabolic side effects are now the subject of long–term studies.

 

 

The Posterior Lobe of the pituitary gland is also called the neurohypophysis, or pars nervosa (nervous part), because it contains the axons of hypothalamic neurons. Neurons of the supraoptic and paraventricular nuclei manufacture antidiuretic hormone (ADH) and oxytocin, respectively. These products move along axons in the infundibulum to the basement membranes of capillaries in the posterior lobe by means of axoplasmic transport.
 
Antidiuretic hormone ( ADH ), also known as arginine vasopressin ( AVP ), is released in response to a variety of stimuli, most notably a rise in the electrolyte concentration in the blood or a fall in blood volume or blood pressure. A rise in the electrolyte concentration stimulates the secretory neurons directly. Because they respond to a change in the osmotic concentration of body fluids, these neurons are called osmoreceptors .

The primary function of ADH is to decrease the amount of water lost at the kidneys. With losses minimized, any water absorbed from the digestive tract will be retained, reducing the concentration of electrolytes in the extracellular fluid. In high concentrations, ADH also causes vasoconstriction , a constriction of peripheral blood vessels that helps elevate blood pressure. ADH release is inhibited by alcohol, which explains the increased fluid excretion that follows the consumption of alcoholic beverages.

 

 

Diabetes occurs in several forms, all characterized by excessive urine production (polyuria). Although diabetes can be caused by physical damage to the kidneys, most forms are the result of endocrine abnormalities. The two most prevalent forms are diabetes mellitus and diabetes insipidus. Diabetes mellitus is described on page 633. Diabetes insipidus generally develops because the posterior lobe of the pituitary gland no longer releases adequate amounts of ADH. Water conservation at the kidneys is impaired, and excessive amounts of water are lost in the urine. As a result, the individual is constantly thirsty, but the fluids consumed are not retained by the body.

 

Mild cases of diabetes insipidus may not require treatment if fluid and electrolyte intake keep pace with urinary losses. In severe cases, the fluid losses can reach 10 liters per day, and dehydration and electrolyte imbalances are fatal without treatment

 

Oxytocin
In women, oxytocin, or OT , stimulates smooth muscle tissue in the wall of the uterus, promoting labor and delivery. After delivery, oxytocin stimulates the contraction of myoepithelial cells around the secretory alveoli and the ducts of the mammary glands, promoting the ejection of milk.

Until the last stages of pregnancy, the uterine smooth muscles are relatively insensitive to oxytocin, but sensitivity becomes more pronounced as the time of delivery approaches. The trigger for normal labor and delivery is probably a sudden rise in oxytocin levels at the uterus. There is good evidence, however, that the oxytocin released at the posterior lobe plays only a supporting role and that most of the oxytocin involved is secreted by the uterus and fetus.

Oxytocin secretion and milk ejection are part of a neuroendocrine reflex. The stimulus is an infant suckling at the breast, and sensory nerves innervating the nipples relay the information to the hypothalamus. Oxytocin is then released into the circulation at the posterior lobe, and the myoepithelial cells respond by squeezing milk from the secretory alveoli into large collecting ducts. This milk let–down reflex can be modified by any factor that affects the hypothalamus. For example, anxiety, stress, and other factors can prevent the flow of milk, even when the mammary glands are fully functional. In contrast, nursing mothers can become conditioned to associate a baby's crying with suckling. These women may begin milk let–down as soon as they hear a baby cry.

Although the functions of oxytocin in sexual activity remain uncertain, it is known that circulating concentrations of oxytocin in both genders rise during sexual arousal and peak at orgasm. Evidence indicates that in men oxytocin stimulates smooth muscle contractions in the walls of the sperm duct ( ductus deferens ) and prostate gland. These actions may be important in emission –the ejection of secretions of the prostate gland, sperm, and the secretions of other glands into the male reproductive tract before ejaculation. Studies suggest that the oxytocin released during intercourse in females may stimulate smooth muscle contractions in the uterus and vagina that promote the transport of sperm toward the uterine tubes.

 

Summary: The Hormones of the Pituitary Gland

 



 

 

 

 

18–4  The Thyroid Gland curves across the anterior surface of the trachea just inferior to the thyroid cartilage , which forms most of the anterior surface of the larynx

 

 

The two lobes of the thyroid gland are united by a slender connection, the isthmus.

 

Thyroid Follicles and Thyroid Hormones

 

The thyroid gland contains large numbers of thyroid follicles , spheres lined by a simple cuboidal epithelium. Follicle cells synthesize a globular protein called thyroglobulin  and secrete it into the colloid of the thyroid follicles. Each thyroglobulin molecule contains the amino acid tyrosine , the building block of thyroid hormones.

 

 

 

The formation of thyroid hormones involves three basic steps:

  1. Iodide ions are absorbed from the diet at the digestive tract and are delivered to the thyroid gland by the bloodstream. Carrier proteins in the basal membrane of the follicle cells transport iodide ions into the cytoplasm. Normally, the follicle cells maintain intracellular concentrations of iodide that are many times higher than those in the extracellular fluid.
  2. The iodide ions diffuse to the apical surface of each follicle cell, where they are converted to an activated form of iodide by the enzyme thyroid peroxidase . This reaction sequence also attaches one or two iodide ions to the tyrosine molecules of thyroglobulin.
  3. Tyrosine molecules to which iodide ions have been attached are paired, forming molecules of thyroid hormones that remain incorporated into thyroglobulin. The pairing process is probably performed by thyroid peroxidase. The hormone thyroxine, also known as tetraiodothyronine , or contains four iodide ions. Triiodothyronine , or is a related molecule containing three iodide ions. Eventually, each molecule of thyroglobulin contains four to eight molecules of or hormones or both.

 

 

The major factor controlling the rate of thyroid hormone release is the concentration of TSH in the circulating blood . TSH stimulates iodide transport into the follicle cells and stimulates the production of thyroglobulin and thyroid peroxidase. TSH also stimulates the release of thyroid hormones. Under the influence of TSH, the following steps occur:

  1. Follicle cells remove thyroglobulin from the follicles by endocytosis.
  2. Lysosomal enzymes break the thyroglobulin down, and the amino acids and thyroid hormones enter the cytoplasm. The amino acids are then recycled and used to synthesize thyroglobulin.
  3. The released molecules of T3 and T4 diffuse across the basement membrane and enter the bloodstream. About 90 percent of all thyroid secretions is is secreted in comparatively small amounts.
  4. Roughly 75 percent of the T4 molecules and 70 percent of the T3 molecules entering the bloodstream become attached to transport proteins called thyroid–binding globulins ( TBGs ). Most of the rest of the T4 and T3 in the circulation is attached to transthyretin , also known as thyroid–binding prealbumin ( TBPA ), or to albumin , one of the plasma proteins. Only the relatively small quantities of thyroid hormones that remain unbound–roughly 0.3 percent of the circulating T3 and 0.03 percent of the circulating T4 –are free to diffuse into peripheral tissues.

 

Functions of Thyroid Hormones
Thyroid hormones readily cross cell membranes, and they affect almost every cell in the body. Inside a cell, they bind to (1) receptors in the cytoplasm, (2) receptors on the surfaces of mitochondria, and (3) receptors in the nucleus.

Thyroid hormones also activate genes that code for the synthesis of enzymes involved in glycolysis and ATP production. This effect, coupled with the direct effect of thyroid hormones on mitochondria, increases the metabolic rate of the cell. Because the cell consumes more energy and because energy use is measured in calories , the effect is called the calorigenic effect of thyroid hormones. When the metabolic rate increases, more heat is generated. In young children, TSH production increases in cold weather; the calorigenic effect may help them adapt to cold climates. (This response does not occur in adults.) In growing children, thyroid hormones are also essential to normal development of the skeletal, muscular, and nervous systems.

 

T3 Versus T4

 

The thyroid gland produces large amounts of T4 but T3 is primarily responsible for the observed effects of thyroid hormones: a strong, immediate, and short–lived increase in the rate of cellular metabolism. Peripheral tissues have two sources of  T3

  1. Release by the Thyroid Gland . At any moment, T3 from the thyroid gland accounts for only 10–15 percent of the in peripheral tissues.
  2. The Conversion of to Enzymes in the liver, kidneys, and other tissues can convert T4 to T3. Roughly 85–90 percent of the T3 that reaches the target cells is produced by the conversion of T4 within peripheral tissues.

 

 

Iodine and Thyroid Hormones
Iodine in the diet is absorbed at the digestive tract. The thyroid follicles contain most of the iodide reserves in the body. The typical diet in the United States provides approximately 500ug of iodide per day, roughly three times the minimum daily requirement. Much of the excess is due to the addition of to the table salt sold in grocery stores as "iodized salt." Thus, iodide deficiency is seldom responsible for limiting the rate of thyroid hormone production. (This is not necessarily the case in other countries.) Excess is removed from the blood at the kidneys, and each day a small amount of (about 20ug ) is excreted by the liver into the bile, an exocrine product stored in the gallbladder. Iodide excreted at the kidneys is eliminated in urine; the iodine excreted in bile is eliminated with intestinal wastes.

 

The C Cells of the Thyroid Gland: Calcitonin
A second population of endocrine cells lies sandwiched between the cuboidal follicle cells and their basement membrane. These cells, which are larger than those of the follicular epithelium and do not stain as clearly, are the C ( clear ) cells , or parafollicular cells. C cells produce the hormone calcitonin ( CT ), which aids in the regulation of  concentrations in body fluids. We introduced the functions of this hormone in Chapter 6 . The net effect of calcitonin release is a drop in the  concentration in body fluids, accomplished by (1) the inhibition of osteoclasts, which slows the rate of release from bone, and (2) the stimulation and then falloff of excretion at the kidneys.

 

Calcitonin is probably most important during childhood, when it stimulates bone growth and mineral deposition in the skeleton. It also appears to be important in reducing the loss of bone mass (1) during prolonged starvation and (2) in the late stages of pregnancy, when the maternal skeleton competes with the developing fetus for calcium ions absorbed by the digestive tract.

 

Thyroid Gland Disorders

 

Normal production of thyroid hormones establishes the background rates of cellular metabolism. These hormones exert their primary effects on metabolically active tissues and organs, including skeletal muscles, the liver, and the kidneys. The inadequate production of thyroid hormones is called hypothy–roidism . In an infant, hypothyroidism produces cretinism , a condition marked by inadequate skeletal and nervous development and a metabolic rate as much as 40 percent below normal levels. The condition affects approximately 1 birth out of every 5000. Cretinism developing later in childhood will retard growth and mental development and delay puberty. Adults with hypothyroidism are lethargic and unable to tolerate cold temperatures. The symptoms of adult hypothyroidism, collectively known as myxedema, include subcutaneous swelling, dry skin, hair loss, low body temperature, muscular weakness, and slowed reflexes. Hypothyroidism may also be associated with the enlargement of the thyroid gland, producing a distinctive swelling called a goiter. Hypothyroidism, myxedema, and goiter as the result of inadequate dietary iodide are very rare in the United States, in part due to the addition of iodine to table salt, but these conditions can be relatively common in poorer countries, especially landlocked ones (seafood is a good source of iodine).

 

 

 

 

 

Thyroid Disorders.
(a) Cretinism, or congenital hypothyroidism, results from thyroid hormone insufficiency in infancy. (b) An enlarged thyroid gland, or goiter, can be associated with thyroid hyposecretion due to iodine insufficiency in adults.

 

Hyperthyroidism , or thyrotoxicosis , occurs when thyroid hormones are produced in excessive quantities. The metabolic rate climbs, and the skin becomes flushed and moist with perspiration. Blood pressure and heart rate increase, and the heartbeat may become irregular as circulatory demands escalate. The effects on the central nervous system make the individual restless, excitable, and subject to shifts in mood and emotional states. Despite the drive for increased activity, the person has limited energy reserves and fatigues easily. Graves' disease is a form of hyperthyroidism that afflicted President George W. H. Bush and Barbara Bush during their stay in the White House.

 

 

 

18–5  The Parathyroid Glands

 There are normally two pairs of parathyroid glands embedded in the posterior surfaces of the thyroid gland

 

 

The parathyroid glands have at least two cell populations: The chief cells produce parathyroid hormone; the functions of the other cells, called oxyphils , are unknown.

Like the C cells of the thyroid gland, the chief cells monitor the circulating concentration of calcium ions. When the  concentration of the blood falls below normal, the chief cells secrete parathyroid hormone( PTH ), or parathormone . The net result of PTH secretion is an increase in  concentration in body fluids. Parathyroid hormone has four major effects:

  1. It stimulates osteoclasts, accelerating mineral turnover and the release of  from bone.
  2. It inhibits osteoblasts, reducing the rate of calcium deposition in bone.
  3. It enhances the reabsorption of  at the kidneys, reducing urinary losses.
  4. It stimulates the formation and secretion of calcitriol at the kidneys. In general, the effects of calcitriol complement or enhance those of PTH, but one major effect of calcitriol is the enhancement of  and  absorption by the digestive tract

 

 

 

 

 

 

 

When the parathyroid gland secretes inadequate or excessive amounts of parathyroid hormone,   concentrations move outside normal homeostatic limits. Inadequate parathyroid hormone production, a condition called hypoparathyroidism , leads to low  concentrations in body fluids. The most obvious symptoms involve neural and muscle tissues: The nervous system becomes more excitable, and the affected individual may experience hypocalcemic tetany , a dangerous condition characterized by prolonged muscle spasms that initially involve the limbs and face.

 

In hyperparathyroidism ,  concentrations become abnormally high. Bones grow thin and brittle, skeletal muscles become weak, CNS function is depressed, and nausea and vomiting occur. In severe cases, the patient may become comatose.