by Jeffrey Dach MD
The TSH Test is perhaps the most important single test mainstream endocrinology relies on for evaluating thyroid function. Surprisingly, TSH, which stands for Thyroid Stimulating Hormone, is not made by the thyroid gland and is not a thyroid hormone. Rather, TSH is made by the pituitary gland, a small nubbin of tissue at the end of a slender stalk attached to the base of the brainstem resting in a bony indentation at the skull base called the sella turcica. Understanding TSH requires an understanding of the HPA, the hypothalamic-
TSH, also called Thyrotropin, is a pituitary glycoprotein consisting of α and β subunits. The α subunit is 92 amino acids in length, and the TSH β (hTSH β) subunit contains 118 amino acids, What is the action of the TSH ? TSH stimulates the thyroid gland to produce more thyroid hormone (thyroxine), like the thermostat in your house which automatically turns on the furnace to make more heat whenever the room temperature falls.
Regulation of TSH-the HPA
TSH production by the pituitary is itself regulated by two things. The first is the level of thyroid hormones in the blood stream which inhibits TSH production by the pituitary, as a negative feedback loop. Secondly. TSH production is stimulated by the action of TRH (Thyrotropin Releasing Horomone), a hormone produced by the hypothalamus in the brainstem. TRH production is itself regulated by thyroid hormone levels in the blood stream, similar to the thermostat analogy above. Increasing thyroid hormone levels inhibits TRH production, and decreasing thyroid hormone levels allows for more TRH production by the hypothalamus. Thus thyroid hormone production by the thyroid gland is part of a feed back loop called the HPA, the hyothalamic pituitary axis.
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