Normal Physiology and Metabolism

The follicular cells reversibly take up iodide ions from the bloodstream against a 30-fold concentration gradient and irreversibly convert them to an organic form, which is secreted into the follicles (Fig. 2). The steps of uptake and organification are stimulated by thyroid stimulating hormone (TSH). Uptake is competitively inhibited by large anions such as thiocyanate and perchlorate. Organification is blocked by the thyroid-blocking drugs, propylthiouracil (PTU) and methimazole. Oxidized iodine is secreted into the colloid where synthesis of mono- and diiodotyrosines occurs, going on to the production of thyroxine (T4) and triiodothyronine (T3). These steps are also blocked by anti-thyroid drugs. Within the follicle, thyroid hormones are bound to thyroglobulin (Tg). The storage capacity of the thyroid as an endocrine gland is exceptional: an average gland contains about 3

Nuclear Medicine, edited by William D. Leslie and I. David Greenberg. ©2003 Landes Bioscience.

Figure 1. Anatomic distribution of lymph nodes that drain the thyroid gland. (From Agur AMR. Grant's Atlas of Anatomy, 9th Ed., 1991. Reproduced with permission from Williams & Wilkins.)
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