Hypothalamus-pituitary-thyroid axis

Fail PA, Anderson S A, Friedman MA (1999) Response of the pituitary and thyroid to tropic hormones in Sprague-Dawley versus Fischer 344 male rats. Toxicol Sci 52 107-121 Kyung OY, Narayanan L, Mattie DR, Godfrey RJ, Todd PN, Sterner TR, Mahle DA, Lumpkin MH, Fisher JW (2002) The Pharmacokinetics of Perchlorate and Its Effect on the Hypothalamus-Pituitary-Thyroid Axis in the Male Rat. Toxicol Applied Pharmacol 182 148-159... 

Hypothalamus-pituitary-thyroid axis and susceptibility to hypertension... 

FIGURE 41-1. Hypothalamic-pituitary-thyroid axis. Thyrotropinreleasing hormone (TRH) is synthesized in the neurons within the paraventricular nucleus of the hypothalamus. TRH is released into the hypothalamic-pituitary portal circulation and carried to the pituitary, where it activates the pituitary to synthesize and release thyrotropin (TSH). TSH activates the thyroid to stimulate the synthesis and secretion of thyroxine (T4) and triiodothyronine (T3). T4 and T3 inhibit TRH and TSH secretion, closing the feedback loop. 

The hypothalamic-pituitary-thyroid axis. Acute psychosis or prolonged exposure to cold may activate the axis. Hypothalamic TRH stimulates pituitary TSH release, while somatostatin and dopamine inhibit it. TSH stimulates T4 and T3 synthesis and release from the thyroid, and they in turn inhibit both TRH and TSH synthesis and release. Small amounts of iodide are necessary for hormone production, but large amounts inhibit T3 and T4 production and release. (Solid arrows, stimulatory influence dashed arrows, inhibitory influence. H, hypothalamus, HP, anterior pituitary.)... 

The components of the hypothalamie-pituitary-thyroid axis are outlined in Figure 2. TRH, a tripeptide, is secreted by the hypothalamus and in turn causes the synthesis of a large glycoprotein hormone. TSH. from the anterior pituitary. This drives the synthesis of thyroid hormones by the thyroid. Production of TSH is regulated by feedback from circulating unbound thyroid hormones. A knowledge of these basics is essential for the correct interpretation of results in the investigation of thyroid disease. Remember ... 

Thyroid hormone production is governed by the actions of the hypothalamic-pituitary-thyroid axis (HPT axis). Thyrotropin releasing hormone (TRH) synthesized by the hypothalamus is transported to the anterior pituitary, where TRF stimulates thyrotrophic cells to produce thyrotrophin (TSH). TSH is a glycoprotein with a short half-life of less than 20 min it is one of three pituitary glycoprotein hormones with alpha- and beta-subunits. These three pituitary glycoproteins (luteotrophin, fol-liotrophin, and TSH) share the same alpha-subunit. 

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... 

Thyroid hormone release is subject to the negative feedback strategy that is typical of endocrine systems controlled by the hypothalamic-pituitary axis. Increased circulating levels of the thyroid hormones (T4, T3) serve to limit their own production by inhibiting TRH release from the hypothalamus and TSH release from the anterior pituitary.30,35 This negative feedback control prevents peripheral levels of thyroid hormones from becoming excessively high. 

The thyroid-pituitary-hypothalamus axis controls thyroid hormone homeostasis. Thyrotropin-releasing hormone (TRH), released from the hypothalamus, stimulates the synthesis and release of thyroid-stimulating hormone (thyrotropin, TSH) from the anterior pituitary. TSH increases the release of thyroid hormones by several mechanisms, including stimulation of the I pump. While lower than normal levels of T3 and T4 cause an exaggerated response of the pituitary to TRH, released thyroid hormones, in feedback control, blunt the stimulating action of TRH on the pituitary. For further discussion of TSH and TRH biochemistry, see, for example, the review by Kannan48. 

The effect of various narcotic analgesics on th5nroid function have been assessed and it was concluded that they did not exert a direct action on the thyroid but depressed thyroid function through an effect on the hypothalamic-pituitary axis. This was further clarified in an elegant study by Lc aax and George whose results lead them to postulate that morphine acts on the thyroid by activating areas in the posterior hypothalamus which inhibit the release of TSH from the pituitary. 

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