Pituitary-thyroid axis

Explain the major components of the hypothalamic-pituitary-thyroid axis and the interaction among these components. 

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. 

There are several ways by which TSH secretion can be increased. An increased hepatic enzyme activity may cause an increased metabolism of thyroid hormones, leading to lower semm hormone levels, which in mm leads to increased secretion of TRH, and subsequently increased TSH secretion. Regarding human relevance, the pathways for regulation of the hypothalamo-pituitary-thyroid axis of rats and humans are similar and the mechanism is relevant for humans, but the human system is far more resistant to perturbation. 

OE219 Al-Quarawi, A. A., M. A. Al-Damegh, and S. A. El-Mougy. Effect of freeze-dried extract of Olea europaea on the pituitary-thyroid axis in rats. Phytother Res 2002 16(3) 286-287. 

Nemeroff CB, Bissette G, Martin JB, et al Effect of chronic treatment with thyrotropin-releasing hormone (TRH) or an analog of TRH (hnear-beta-alanine TRH) on the hypothalamic-pituitary-thyroid axis. Neuroendocrinology 30 193-199, 1980... 

Dysregulation of the hypothalamic-pituitary-thyroid axis causes a reduction in thyroid function. There may be a relationship between an abnormal TSH response to TRH and depressive symptoms. Thus, unipolar patients undergoing the TRH-TSH test (which measures the difference between baseline TSH and peak postinfusion TSH after they are given synthetic TRH) reportedly have a blunted response, whereas bipolar, depressed patients have an elevated response (see also Chapter 1,... 

The hypothalamic-pituitary-thyroid axis. Acute psychosis or prolonged exposure to cold may activate the axis. Hypothalamic thyroidreleasing hormone (TRH) stimulates pituitary thyroid-stimulating hormone (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. 

Allen-Rowlands CP, Castracane VD, Hamilton MG, et al. 1981. Effect of polybrominated biphenyls (PBB) on the pituitary - thyroid axis of the rat. Proc Soc Exp Biol Med 166 506-514. 

The many effects of lithium on thyroid physiology and on the hypothalamic-pituitary axis and their clinical impact (goiter, hypothyroidism, and hyperthyroidism) have been reviewed (620). Lithium has a variety of effects on the hypothalamic-pituitary-thyroid axis, but it predominantly inhibits the release of thyroid hormone. It can also block the action of thyroid stimulating hormone (TSH) and enhance the peripheral degradation of thyroxine (620). Most patients have enough thyroid reserve to remain euthyroid during treatment, although some initially have modest rises in serum TSH that normalize over time. 

Nikrodhanond AA, Ortiga-Carvalho TM, Shibusawa N, et al. Dominant role of thyrotropin-releasing hormone in the hypothalamic-pituitary-thyroid axis. J Biol Chem. 2006 281 5000-5007. 

Pituitary-thyroid axis Pituitary weight Thyroid weight... 

Faber J, Kirkegaard C, Rasmussen B, Westh H et al. (1987) Pituitary-thyroid axis in critical illness. J Clin Endocrinol Metab 65 315... 

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... 

Khan MA, Fenton SE, Swank AE, et al. A mixture of ammonium perchlorate and sodium chlorate enhances the pituitary-thyroid axis caused by the individual chemicals in adult male F344 rats. Toxicol Pathol 2005 33(7) 776-83. 

When the serum free thyroxine concentration (FT4) or ultrasensitive TSH result is normal, the hypothalamic-pituitary-thyroid axis is assumed to be intact. If primary hypothyroidism is suspected clinically, however, a single measurement of a basal TSH concentration may be sufficient to confirm the diagnosis. In patients with a history of pituitary disease and secondary hypothyroidism, the serum TSH concentration is frequently normal thus in this situation, an FT4 concentration is tlie better test to gauge normality of the hypothalamic-pituitary-thyroid axis. On occasion a TRH stimulation test is used to distinguish hypothalamic... 

Figure 52-2 Hypothalamic-pituitary-thyroid axis—hormone synthesis dependent on dietary intake of ISO pg of iodine per day.T4 major thyronine secreted from thyroid gland with T3 coming predominantly from peripheral deiodination.

T. Kimura, T. Kanda, A. Kuwabara, H. Shinohara, I. Kobayashi, Participation of the pituitary-thyroid axis in the cardiovascular systemic elderly patients with congestive heart failure, J Med 28, 75-80 (1997). 

Pituitary-Thyroid Axis and T3 production [61, 64], TSH production is stimulated by the... 

Jennings AS, Schwartz SL, Balter NJ, Gardner D, Witorsch RJ (1990) Effects of oral erythro-sine (2, 4, 5, 7 -tetraiodofluorescein) on the pituitary-thyroid axis in rats. Toxicol Appl Pharmacol 103(3) 549-556... 

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