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Thyroid Physiology I

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BIOL 1840U
Peter Cheung

Thyroid Physiology I. Introduction a. Anatomy rd i. Generally bilobed, but some people have small 3 lobe (pyramidal lobe) II. The thyroid gland synthesizes, stores, & secretes the thyroid hormones— principally thyroxine (T4) & to a lesser extent triiodothyronine (T3) a. Biochemistry & hormonogenesis i. iodine trapped by Na-I Cotransporter in cell membrane ii. iodine transported to apical membrane (via transporter) iii. thyroglobulin produced & stored in follicle iv. thyroid peroxidase (TPO) ionizes iodine & links it to thyroglobulin covalently = Organification v. this => monoiodotyrosine; diodotyrosine, etc vi. Coupling = 2 tyrosines (w/ iodine) are linked together => thyroxine (T4) vii. pinocytosis & transport of thyroid hormones across cell viii. secretion at basal membrane III. Dietary iodine is a substrate for thyroid hormone synthesis, but in larger amounts it can inhibit thyroid hormone production (autoregulation) a. iodine is trapped, oxidized, & covalently bound to tyrosines in thyroglobulin => vital in thyroid hormone production b. Iodine deficiencies i. ↑ fractional iodine uptake by the thyroid ii. ↑ thyroid gland size (goiter) iii. hypothyroidism iv. Cretinism—iodine deficiency during gestation => irreversible brain damage c. Autoregulation i. lg amounts of iodine => inhibition of thyroid hormone production/release ii. thyroid gland can “escape” from the inhibitory effects of ↑ iodine after 2 weeks iii. combination of these factors allow for uncoupling of T3/T4 production from the amount of iodine ingested daily IV. Thyroid gland function is controlled by hypothalamic TRH and pituitary TSH which are in turn regulated by the negative feedback of thyroid hormones a. TSH (secr
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