CHAPTER 18 Endocrine Glands
615
Regulation of Thyroid Hormone Secretion
Thyrotropin-releasing hormone (TRH) from the hypothalamusand TSH from the anterior pituitary function together to increaseT
3
and T
4
secretion from the thyroid gland (figure 18.10). Stressand exposure to cold cause increased TRH secretion, and pro-longed fasting decreases TRH secretion. TRH stimulates thesecretion of TSH from the anterior pituitary. TSH travels to thethyroid gland, where it stimulates the synthesis and secretion ofT
3
and T
4
. TSH also causes hypertrophy (increased cell size) andhyperplasia (increased cell number) of the thyroid gland.Decreased blood levels of TSH lead to decreased T
3
and T
4
secre-tion and to thyroid gland atrophy. T
3
and T
4
have a negative-feedback effect on the hypothalamus and anterior pituitary gland.As T
3
and T
4
levels increase in the blood, they inhibit TRH andTSH secretion. If the thyroid gland is removed or if T
3
and T
4
secretion declines, TSH levels in the blood increase dramatically.Conditions in which TSH is elevated can often be charaterized bythe abnormal thyroid gland overgrowth called goiter (table 18.5)Hypothyroidism, or reduced secretion of thyroid hormones,can result from having an iodine deficiency, taking certain drugs,or being exposed to chemicals that inhibit T
3
and T
4
synthesis. Itcan also be caused by inadequate secretion of TSH, by an autoim-mune disease that depresses thyroid hormone function, or by sur-gical removal of the thyroid gland. Hypersecretion of T
3
and T
4
can result from the synthesis of an immunoglobulin that stimulatesTSH receptors and acts like TSH, from TSH-secreting tumors ofthe pituitary gland, and from thyroid tumors.
Interestingly, complete thyroidectomy does not result in high
+ +
blood Ca
2
levels, possibly because the regulation of blood Ca
2
levels by vitamin D and other hormones, such as parathyroid hor-mone (if the parathyroid glands are retained in the body), compen-sates for the loss of calcitonin in individuals who have undergonea thyroidectomy. No pathological condition is directly associatedwith a lack of calcitonin secretion. Some evidence suggests thatcalcitonin may play a role in regulating food intake by decreasingappetite. Clinically, calcitonin nasal sprays have been effective inthe management of postmenopausal osteoporosis.
Predict 5
Becky has lost 30 pounds over the past several months, even though herappetite has been good and she has been eating more than usual. Shecomplains to her physician that she is nervous and restless, has a shortattention span, becomes fatigued easily but cannot sleep well, movescompulsively, and sweats excessively. Her physician notes that she alsoexhibits tachycardia. Suspecting hyperthyroidism, he orders a blood test,which indicates elevated levels of T
3
and T
4
and low levels of TSH. Beckyalso has a TSH-like immunoglobulin in her plasma. Explain these results.
Calcitonin
Parafollicular cells, or C cells, are dispersed between the thyroidfollicles throughout the thyroid gland. These cells secrete thehormone calcitonin in response to increased calcium levels in theblood.The primary target tissue for calcitonin is bone (see chapter 6).Calcitonin binds to membrane-bound receptors, decreases osteo-clast activity, and lengthens the life span of osteoblasts. The result-ing bone deposition leads to decreases in blood calcium andphosphate levels.
+
Calcitonin’s importance in regulating blood Ca
2
levels isunclear; it may be most important in juveniles in promoting bonegrowth. The rate of calcitonin secretion increases in response to
+
elevated blood Ca
2
levels, and calcitonin may prevent large
+
increases in blood Ca
2
levels following a meal. Blood levels ofcalcitonin decrease with age to a greater extent in females than inmales. Also, the incidence of osteoporosis increases with age andoccurs to a greater degree in females than in males.