Thyroid Factor

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1. As a result of stress, nerve impulses are transmitted to the hypothalamus.

2. Sympathetic impulses arising from the hypothalamus increase blood glucose concentration, blood glycerol concentration, blood fatty acid concentration, heart rate, blood pressure, and breathing rate. They dilate air passages, shunt blood into skeletal muscles, and increase secretion of epinephrine from the adrenal medulla.

3. Epinephrine intensifies and prolongs sympathetic actions.

4. The hypothalamus secretes CRH, which stimulates secretion of ACTH by the anterior pituitary gland.

5. ACTH stimulates release of cortisol by the adrenal cortex.

6. Cortisol increases the concentration of blood amino acids, releases fatty acids, and forms glucose from noncarbohydrate sources.

7. Secretion of glucagon from the pancreas and growth hormone from the anterior pituitary increase.

8. Glucagon and growth hormone aid mobilization of energy sources and stimulate uptake of amino acids by cells.

9. Secretion of ADH from the posterior pituitary increases.

10. ADH promotes the retention of water by the kidneys, which increases blood volume.

11. Renin increases blood levels of angiotensin II, which acts as a vasoconstrictor and also stimulates aldosterone secretion by the adrenal cortex.

12. Aldosterone stimulates sodium retention by the kidneys.

Levels of antidiuretic hormone increase with age, but this is due to slowed breakdown in the liver and kidneys, rather than increased synthesis. As a result, the kidneys are stimulated to reabsorb more water.

The thyroid gland shrinks with age, as individual follicles shrink and become separated by increasing amounts of fibrous connective tissue. Nodules, which may be benign or cancerous, become more common with age. Upon autopsy, many individuals are found to have thyroid nodules that were never detected. Although blood levels of T3 and T4 may diminish with age, in general, the thyroid gland's control over the metabolism of various cell types is maintained throughout life. Calci-tonin levels decline with age, which raises the risk of osteoporosis. Decrease in estrogen levels in post-menopausal women exacerbates the calcitonin loss and associated elevated osteoporosis risk.

Parathyroid function differs between the sexes with age. Secretion peaks in males at about age fifty, whereas in women, the level of parathyroid hormone decreases until about age forty, after which it rises and contributes to osteoporosis risk. Fat accumulates between the cells of the parathyroid glands.

The adrenal glands illustrate the common theme of aging-related physical changes, yet continued function. Fibrous connective tissue, lipofuscin pigment, and increased numbers of abnormal cells characterize the aging adrenal glands. However, thanks to the fine-tuning of negative feedback systems, blood levels of glucocorti-coids and mineralocorticoids usually remain within the normal range, although the ability to maintain homeosta-sis of osmotic pressure, blood pressure, acid/base balance and sodium and potassium ion distributions may falter with age.

The most obvious changes in endocrine function that occur with age involve blood glucose regulation. The pancreas may be able to maintain secretion of insulin and glucagon, but lifestyle changes, such as increase in fat intake and less exercise, may lead to an increase in blood insulin level. The development of insulin resistance—the decreased ability of muscle, liver and fat cells to take in glucose even in the presence of insulin—reflects impaired ability of these target cells to respond to the hormone, rather than compromised pancreatic function. Blood glu cose buildup may signal the pancreas to secrete more insulin, setting the stage for type II diabetes mellitus.

The daily fall and rise of melatonin levels may even out somewhat with age, which may alter control of the sleep/wake cycle. Changes to the tempo of the body clock may in turn affect secretion of other hormones.

The thymus gland begins to noticeably shrink before age twenty, with accompanying declining levels of thymosins. By age sixty, thymosin secretion is nil. The result is a slowing of the maturation of B and T cells, which accounts for increased susceptibility to infections with age.

1. What general types of changes occur in the glands of the endocrine system with aging?

2. How do the structures and functions of particular endocrine glands change over a lifetime?

Clinical Terms Related to the Endocrine System adrenalectomy (ah-dre"nah-lek'to-me) Surgical removal of the adrenal glands.

adrenogenital syndrome (ah-dre"no-jen'i-tal sin'drom) Group of symptoms associated with changes in sexual characteristics as a result of increased secretion of adrenal androgens.

exophthalmos (ek"sof-thal'mos) Abnormal protrusion of the eyes.

hirsutism (her'sut-izm) Excess hair growth, especially in women.

hypercalcemia (hi"per-kal-se'me-ah) Excess blood calcium. hyperglycemia (hi"per-gli-se'me-ah) Excess blood glucose. hypocalcemia (hi"po-kal-se'me-ah) Deficient blood calcium. hypophysectomy (hi-pof"i-sek'to-me) Surgical removal of the pituitary gland. parathyroidectomy (par"ah-thi"roi-dek'to-me) Surgical removal of the parathyroid glands. pheochromocytoma (fe-o-kro"mo-si-to'mah) Type of tumor found in the adrenal medulla and usually accompanied by high blood pressure. polyphagia (pol"e-fa'je-ah) Excessive eating. thymectomy (thi-mek'to-me) Surgical removal of the thymus gland.

thyroidectomy (thi"roi-dek'to-me) Surgical removal of the thyroid gland.

thyroiditis (thi"roi-di'tis) Inflammation of the thyroid gland.

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