Overview
The Physiology Behind the Result
Thyroid stimulating hormone (TSH) is secreted by the anterior pituitary after stimulation by hypothalamic thyrotropin releasing hormone.
CNPLE
CNPLE
Thyroid-stimulating hormone (TSH) is secreted by the anterior pituitary after stimulation by hypothalamic thyrotropin-releasing hormone. TSH signals the thyroid gland to produce thyroid hormone, principally thyroxine (T4) and triiodothyronine (T3). Circulating free T4 and T3 feed back to the hypothalamus and pituitary, reducing TRH and TSH release. The pituitary responds log-linearly to free T4. A small fall in free T4 can therefore produce a much larger rise in TSH; a small rise in free T4 can markedly suppress TSH. This amplified response makes TSH the most sensitive single indicator of primary thyroid dysfunction. It is a signal of the pituitary's response to thyroid hormone, not a direct measurement of thyroid hormone action in every tissue. That distinction explains the usual outpatient approach: begin with TSH. A result within the reporting laboratory's reference interval excludes the great majority of primary thyroid disorders. It does not, however, exclude pituitary or hypothalamic disease, acute non-thyroidal illness, or a result distorted by assay interference.
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