Iodine is essential for the formation of thyroxin. Simple goiter, a diffuse and painless enlargement of the thyroid gland, was once common in areas of the United States where the iodine content of the soil and water   was   inadequate.   In   simple   goiter,   the   gland enlarges   to   compensate   for   the   lack   of   iodine.   To prevent   formation   of   a   simple   goiter,   iodine- containing foods, such as vegetables, iodized salt, and seafood, should be eaten. A condition known as  hypothyroidism  is caused by  an  insufficient  secretion  of  thyroxin.  The  patient exhibits a decrease in basal metabolism, and sweating is  almost  absent.  There  may  be  a  weight  gain  and constant  fatigue.  The  heart  rate  may  be  slow,  and  a simple goiter may form. There may also be personality changes   characterized   by   slow,   lethargic   mental functioning.   Hypothyroidism   during   childhood   can lead to the development of  cretinism. Cretinism is a condition   characterized   by   retarded   mental   and physical development. PARATHYROID GLANDS Parathyroid  glands  are  four  small  round  bodies located just posterior to the thyroid gland (fig. 1-47). Their hormone,  parathormone  (PTH), regulates the calcium   and   phosphorus   content   of   the   blood   and bones. The amount of calcium is important in certain tissue activities, such as bone formation, coagulation of blood, maintenance of normal muscular excitability, and milk production in the nursing mother. Diminished function or removal of the parathyroid glands results in a  low  calcium  level  in  the  blood.  In  extreme  cases death may occur, preceded by strong contraction of the muscles (tetany) and convulsions. Hyperparathyroidism, an excess of parathyroid hormone  in  the  blood,  causes  calcium  levels  in  the blood   to   become   elevated   by   the   withdrawal   of calcium   from   the   bones,   leaving   the   skeleton demineralized  and  subject  to  spontaneous  fractures. The excess calcium may be deposited as stones in the kidneys. ADRENAL GLANDS The  adrenal  glands  are  located  on  the  superior surface  of  each  kidney,  fitting  like  a  cap  (fig.  1-47). They  consist  of  an  outer  portion,  the  cortex,  and  an inner portion, the medulla. Adrenal Cortex Specialized cells in the outer layer of the adrenal cortex produce three types of steroid hormones that are of vital importance. MINERALOCORTICOIDS.—Mineralocor- ticoids are regulators of fluid and electrolyte balance. They  are  sometimes  called  salt  and  water  hormones because they regulate the excretion and absorption of sodium, chlorine, potassium, and water. GLUCOCORTICOIDS.—Glucocorticoids   are essential  to  metabolism.  They  increase  certain  liver functions   and   have   an   anti-inflammatory   effect. Clinically,   they   are   used   to   suppress   inflammatory reactions, to promote healing, and to treat rheumatoid arthritis. ANDROGENS   AND   ESTROGENS.—The adrenal cortex also produces sex hormones, some with male characteristics (androgens), others with female characteristics (estrogens). These hormones appear in different concentrations in both men and women. Adrenal Medulla The   adrenal   medulla   secretes   epinephrine (adrenalin)   in   the   presence   of   emotional   crises, hypoglycemia   (low   blood   sugar),   or   low   blood pressure. Epinephrine causes powerful contractions of many   arterioles   (especially   in   the   skin,   mucous membranes, and kidneys), but it dilates other arterioles (such   as   those   of   the   coronary   system,   skeletal muscles,  and  lungs).  Heart  rate,  respiration  rate  and depth,   blood   pressure,   blood   sugar   levels,   and metabolism  are  all  increased  by  epinephrine.  It  also stimulates   the   production   of   other   adrenal   cortical hormones. Norepinephrine  is  also  produced  in  the  adrenal medulla. It is a chemical precursor to epinephrine. Its effects are similar to those of epinephrine, but its action differs. Despite  these  marked  influences,  the  medullary tissue  of  the  adrenal  gland  is  not  essential  to  life, because its various functions can be assumed by other regulatory mechanisms. PANCREAS The   pancreas   contains   two   types   of   secretory tissues.  The  first  secretory  tissue  secretes  digestive juice through a duct to the small intestine, while the other tissue releases hormones into body fluids. The 1-43