The evaluation of patients with abnormal thyroid function test results is one of the major reasons for endocrinology consultations requested by nonendocrine physicians. A review of basic thyroid pathophysiology and the "euthyroid sick syndrome" will be discussed herein to assist all physicians in evaluating thyroid function.
The system regulating thyroid hormone secretion is designed to provide a relatively constant flow of hormones to all tissues of the body. In contrast to the cells secreting pancreatic, parathyroid, and intestinal hormones, the thyroid gland does not respond to varying levels of circulating chemicals or nonhormonal metabolites, but rather is controlled by the pituitary thyrotropin, thyroid-stimulating hormone (TSH).
Thyroid-stimulating hormone secretion is mediated by hypothalamic thyrotropin releasing hormone (TRH), a simple tripeptide that reaches the pituitary via the portal veins that course the pituitary stalk. The negative feedback essential for maintaining euthyroidism is not from the level of circulating thyroid hormone to the
Landau RL. Endocrinology. JAMA. 1985;254(16):2277–2278. doi:10.1001/jama.1985.03360160109024
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