Thyroid Physiology Metabolic regulation Iodine transport n NaI

Thyroid Physiology Metabolic regulation

Iodine transport n Na+/I- symport protein controls serum I- uptake n Based on Na+/K+ antiport potential n Stimulated by TSH n Inhibited by Perchlorate

Thyroid hormone formation n Thyroid Peroxidase (TPO) n Iodine coupled to Thyroglobulin n Pre-hormones secreted into follicular space – Apical membrane protein – Catalyzes Iodine organification to tyrosine residues of thyroglobulin – Monoiodotyrosine (Tg + one I-) – Diiodotyrosine (Tg + two I-)

Iodine enter Follicular cell of thyroid gland TSH as stimulator

Wolff-Chaikoff Effect n n Increasing doses of Iincrease hormone synthesis initially Higher doses cause cessation of hormone formation. This effect is countered by the Iodide leak from normal thyroid tissue. Patients with autoimmune thyroiditis may fail to adapt and become hypothyroid.

Jod-Basedow Effect Opposite of the Wolff-Chaikoff effect n Excessive iodine loads induce hyperthyroidism n Observed in hyperthyroid disease processes n – – – n Graves’ disease Toxic multinodular goiter Toxic adenoma This effect may lead to symptomatic thyrotoxicosis in patients who receive large iodine doses from – – – Dietary changes Contrast administration Iodine containing medication (Amiodarone)

Thyroid Hormone Control

TRH n n Produced by Hypothalamus Downregulated by T 3 Travels through portal venous system Stimulates TSH formation

TSH n n n Produced by Adenohypophysis Thyrotrophs Upregulated by TRH Downregulated by T 4, T 3 Travels through portal venous system to cavernous sinus, body. Stimulates several processes – – – Iodine uptake Colloid endocytosis Growth of thyroid gland

Thyroid Hormone n Majority of circulating hormone is T 4 n Total Hormone load is influenced by serum binding proteins – – – n 98. 5% T 4 1. 5% T 3 Albumin 15% Thyroid Binding Globulin 70% Transthyretin 10% Regulation is based on the free component of thyroid hormone

Thyroid Evaluation n n n TRH TSH Total T 3, T 4 Free T 3, T 4 Thyroglobulin Antibodies: Anti-TPO, Anti-TSHr

Thyroid Evaluation

Iodine states RAIU: Scintillation counter measures radioactivity after I 123 administration. n Normal Thyroid n Inactive Thyroid n Hyperactive Thyroid