Hypothalamic-pituitary-thyroid regulation

Thyroliberin, thyrotropin-releasing hormone, TRH, pGlu-His-Pro-NH2, produced in the paraventricular nucleus of the hypothalamus, stimulates biosynthesis and secretion of thyrotropin (TSH) from the anterior pituitary. It is central in regulating the hypothalamic-pituitary-thyroid (HPT) axis. Furthermore, TRH influences the release of other hormones, e.g., prolactin, growth hormone, vasopressin, insulin, and also the classic neurotransmitters norepinephrine (noradrenaline) and epinephrine (adrenaline). In addition, it is... 

Strbak, V. and Greer, M.A. (1979) Acute effects of hypothalamic ablation on plasma thyrotropin and prolactin concentrations in the suckling rat Evidence that early postnatal pituitary-thyroid regulation is independent of hypothalamic control. Endocrinology 105 488. 

Fig. 6. Hormones of the hypothalamic-pituitary axis and the glands and target organs they regulate (ACTH = adrenocorticotropic hormone LH = luteinizing hormone FSH = follicle stimulating hormone TSH = thyroid stimulating hormone PRL = prolactin GH = growth hormone).

Thyroid hormones enter the extrathyroidal tissues by the action of specific transporters the organic anion transport protein (OATP) and the monocarboxylic transporter (MCT8) (Ekins et al. 1994 Friesema et al. 1999, 2005). Within some cells, deiodinating enzymes can remove an iodine atom from the T4 or T3 molecules and thereby modify the sensitivity of a cell to the actions of the thyroid hormones. The cellular actions of the thyroid hormones are mediated by nuclear receptor proteins, which bind to DNA and regulate transcription with at least two types of genes that encode these receptors (Barettino, Ruiz, and Stunnenberg 1994) these also provide a negative feedback action on the hypothalamic-pituitary axis. T4 is sometimes... 

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. 

Regulation of synthesis Thyroid function is controlled by a tropic hormone, thyroid-stimulating hormone (TSH, thyrotropin), a glycoprotein synthesized by the anterior pituitary (see Figure 25.2). TSH generation is governed by the hypothalamic thyrotropin-... 

Regulation of secretion Secretion of TSH by the anterior pituitary is stimulated by the hypothalamic TRH. Feedback inhibition of both TRH and TSH secretion occurs with high levels of circulating thyroid hormone or iodide. Most of the hormone (T3 and T4) is bound to thyroxine-binding globulin in the plasma. 

FIGURE 56-5 Regulation of thyroid hormone secretion. Myriad neural inputs influence hypothalamic secretion of thyrotropin-releasing hormone (TRH). TRH stimulates release of thyrotropin (TSH, thyroid-stimulating hormone) from the anterior pituitary TSH stimulates the synthesis and release of the thyroid hormones and T,. and T, feed back to inhibit the synthesis and release of TRH and TSH. Somatostatin (SST) can inhibit TRH action, as can dopamine and high concentrations of glucocorticoids. Low levels of L are required for thyroxine synthesis, but high levels inhibit thyroxine synthesis and release. 

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