Thyroid hormone secretion control

Feek CM, Sawers JS, Brown NS, Seth J, Irvine WJ, Toft AD. Influence of thyroid status on dopaminergic inhibition of thyrotropin and prolactin secretion evidence for an additional feedback mechanism in the control of thyroid hormone secretion. J Clin Endocrinol Metab 1980 51(3) 585-9. 

Q3 The following diagram shows the control of thyroid hormone secretion ... 

Q10 Many endocrine secretions are controlled by negative feedback systems. When the thyroid is stimulated and thyroid hormone concentration increases, it inhibits production of TSH to reduce further stimulation of the gland. As thyroid hormone secretion then diminishes, the negative feedback on the anterior pituitary is reduced and TSH secretion increases again. Basically, in... 

The secretion of thyroid hormones is controlled primarily by TSH from the anterior pituitary. 

Effect of Thyroid Hormones 445 Hyperthyroidism 451 Pathology and Symptoms Control of Thyroid Hormone Secretion Thyrotropin... 

Vagenalds, A.G., Downs, P., Braverman, L.E., et al. Control of thyroid hormone secretion in normal subjects receiving iodides, J. Clin. Invest. 52 528 (1973). 

Hormonal actions occur during sensitive periods in development, in adult life during natural endocrine cycles and in response to experience as well as during the aging process (see Ch. 30). As a result of their fundamental actions on cellular processes and genomic activity and of the control of their secretion by environmental signals, steroid and thyroid hormone actions on the brain provide unique insights into the plasticity of the brain and behavior (see also Ch. 50). 

Thyroid hormone production is regulated by TSH secreted by the anterior pituitary, which in turn is under negative feedback control by the circulating level of free thyroid hormone and the positive influence of hypothalamic thyrotropin-releasing hormone. Thyroid hormone production is also regulated by extrathyroidal deiodination of T4 to T3, which can be affected by nutrition, nonthyroidal hormones, drugs, and illness. 

Trace amounts of iodine are required for a healthy body. Iodine is part of the hormone thyroxin produced by the thyroid gland. Thyroid secretions control the physical and mental development of the human body. A goiter, a swelling of the thyroid gland, is caused by the lack of iodine. Adding thyroid medication and iodized salt to the diet helps prevent this disease. Radioactive iodine (1-131), with a half-life of eight days, is used to treat some diseases of the thyroid gland. 

Thyroid hormones accelerate metabolism. Their release (A) is regulated by the hypophyseal glycoprotein TSH, whose release, in turn, is controlled by the hypothalamic tripeptide TRH. Secretion of TSH declines as the blood level of thyroid hormones rises by means of this negative feedback mechanism, hormone production is automatically adjusted to demand. 

The secretion of anterior pituitary hormones is controlled in part by hypothalamic regulatory factors that are stored in the hypothalamus and are released into the adenohypophyseal portal vasculature. Hypothalamic regulatory factors so far identified are peptides with the exception of dopamine. Secretion of anterior pituitary hormones is also controlled by factors produced more distally that circulate in the blood. Predominant control of hormone production may be relatively simple, as with thyroid-stimulating hormone (TSH), the production of which is primarily stimulated by thyrotropin-releasing hormone (TRH) and inhibited by thyroid hormones, or it may be complex, as with prolactin, the production of which is affected by many neurotransmitters and hormones. 

In addition, the metabohsm of OCAs results in the release of large amounts of E into the circulation. As described for KI, I released from OCAs may have effects at the thyroid gland and if used alone to treat hyperthyroidism, OCAs carry the same potential to induce increased secretion of thyroid hormone and exacerbation of thyrotoxicosis. When an OCA is used in the treatment of hyperthyroidism, large doses of antithyroid agents are usually administered concomitantly. However, the combination of OCAs and antithyroid drugs may cause resistance to the antithyroid drugs with time, presumably because of the elevation in intrathyroidal 1 content. Thus, it is recommended that the use of OCAs be reserved for short-term treatment of patients with severe thyrotoxicosis and significant comorbidity (e.g., myocardial infarction, sepsis, stroke) for rapid control of plasma Tj concentrations. 

Lithium inhibits thyroidal incorporation of L into Tg, as well as the secretion of thyroid hormones, but it does not inhibit the activity of the Na+-I symporter or the accumulation of I within the thyroid. Lithium offers no particular advantage over drugs of the thionamide class but may be employed for temporary control of thyrotoxicosis in patients who are allergic to both thion-amides and iodide. 

Control of thyroid function via thyroid-pituitary feedback is also discussed in Chapter 37. Briefly, hypothalamic cells secrete thyrotropin-releasing hormone (TRH) (Figure 38-3). TRH is secreted into capillaries of the pituitary portal venous system, and in the pituitary gland, TRH stimulates the synthesis and release of thyrotropin (thyroid-stimulating hormoneTSH). TSH in turn stimulates an adenylyl cyclase-mediated mechanism in the thyroid cell to increase the synthesis and release of T4 and T3. These thyroid hormones act in a negative feedback fashion in the pituitary to block the action of TRH and in the hypothalamus to inhibit the synthesis and secretion of TRH. Other hormones or drugs may also affect the release of TRH or TSH. 

Dietary vitamin A is stored in the liver and secreted into the bloodstream when needed. The circulating retinol is taken up by target cells and oxidized in part to retinoic acid, which induces the synthesis of proteins through the direct control of gene expression. This type of action—gene activation—establishes vitamin A (in the form of its metabolite, retinoic acid) as a hormone, similar to the steroid hormones and the thyroid hormone. 

The thyroid and parathyroid glands serve a number of vital endocrine functions. The thyroid gland synthesizes and secretes the thyroid hormones T3 and T4. These hormones are important regulators of cellular metabolism and metabolic rate. Thyroid hormones also interact with other hormones to facilitate normal growth and development. The parathyroid glands control calcium homeostasis through the release of PTH. This hormone is crucial... 

Q1 Thyroid secretion is controlled by two feedback loops. Secretion of T3 and T4 is stimulated by TSH from the anterior pituitary gland. The secretion of TSH is controlled by the hypothalamus via production of TRH. TRH is secreted by the hypothalamus into the hypophyseal portal blood flow. Stimuli such as a very cold environment influence the secretion of thyroid hormones by affecting the hypothalamus and increasing the release of TRH. 

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