Iodine is essential for the formation of thyroxin.Simple goiter, a diffuse and painless enlargement ofthe thyroid gland, was once common in areas of theUnited States where the iodine content of the soil andwater was inadequate. In simple goiter, the glandenlarges to compensate for the lack of iodine. Toprevent formation of a simple goiter, iodine-containing foods, such as vegetables, iodized salt, andseafood, should be eaten.A condition known as hypothyroidism is causedby an insufficient secretion of thyroxin. The patientexhibits a decrease in basal metabolism, and sweatingis almost absent. There may be a weight gain andconstant fatigue. The heart rate may be slow, and asimple goiter may form. There may also be personalitychanges characterized by slow, lethargic mentalfunctioning. Hypothyroidism during childhood canlead to the development of cretinism. Cretinism is acondition characterized by retarded mental andphysical development.PARATHYROID GLANDSParathyroid glands are four small round bodieslocated just posterior to the thyroid gland (fig. 1-47).Their hormone, parathormone (PTH), regulates thecalcium and phosphorus content of the blood andbones. The amount of calcium is important in certaintissue activities, such as bone formation, coagulationof blood, maintenance of normal muscular excitability,and milk production in the nursing mother. Diminishedfunction or removal of the parathyroid glands results ina low calcium level in the blood. In extreme casesdeath may occur, preceded by strong contraction of themuscles (tetany) and convulsions.Hyperparathyroidism, an excess of parathyroidhormone in the blood, causes calcium levels in theblood to become elevated by the withdrawal ofcalcium from the bones, leaving the skeletondemineralized and subject to spontaneous fractures.The excess calcium may be deposited as stones in thekidneys.ADRENAL GLANDSThe adrenal glands are located on the superiorsurface of each kidney, fitting like a cap (fig. 1-47).They consist of an outer portion, the cortex, and aninner portion, the medulla.Adrenal CortexSpecialized cells in the outer layer of the adrenalcortex produce three types of steroid hormones that areof vital importance.MINERALOCORTICOIDS.—Mineralocor-ticoids are regulators of fluid and electrolyte balance.They are sometimes called salt and water hormonesbecause they regulate the excretion and absorption ofsodium, chlorine, potassium, and water.GLUCOCORTICOIDS.—Glucocorticoids areessential to metabolism. They increase certain liverfunctions and have an anti-inflammatory effect.Clinically, they are used to suppress inflammatoryreactions, to promote healing, and to treat rheumatoidarthritis.ANDROGENS AND ESTROGENS.—Theadrenal cortex also produces sex hormones, some withmale characteristics (androgens), others with femalecharacteristics (estrogens). These hormones appear indifferent concentrations in both men and women.Adrenal MedullaThe adrenal medulla secretes epinephrine(adrenalin) in the presence of emotional crises,hypoglycemia (low blood sugar), or low bloodpressure. Epinephrine causes powerful contractions ofmany arterioles (especially in the skin, mucousmembranes, and kidneys), but it dilates other arterioles(such as those of the coronary system, skeletalmuscles, and lungs). Heart rate, respiration rate anddepth, blood pressure, blood sugar levels, andmetabolism are all increased by epinephrine. It alsostimulates the production of other adrenal corticalhormones.Norepinephrine is also produced in the adrenalmedulla. It is a chemical precursor to epinephrine. Itseffects are similar to those of epinephrine, but its actiondiffers.Despite these marked influences, the medullarytissue of the adrenal gland is not essential to life,because its various functions can be assumed by otherregulatory mechanisms.PANCREASThe pancreas contains two types of secretorytissues. The first secretory tissue secretes digestivejuice through a duct to the small intestine, while theother tissue releases hormones into body fluids. The1-43
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