A 45-year-old woman with differentiated thyroid carcinoma post-thyroidectomy is treated with radioiodine (I-131). The mechanism by which thyroid follicular cells (and metastatic deposits) concentrate I-131 is:

• A Passive diffusion across the follicular cell membrane
• B Active transport via the sodium-iodide symporter (NIS) ✓
• C Receptor-mediated endocytosis via TSH receptors
• D Non-specific pinocytosis by follicular cells

Explanation

The sodium-iodide symporter (NIS) is a membrane glycoprotein expressed on the basolateral surface of thyroid follicular cells that actively transports iodide against its electrochemical gradient, coupled to the inward transport of two sodium ions down their gradient. Well-differentiated thyroid carcinomas retain NIS expression, enabling radioiodine concentration and therapeutic ablation with I-131. TSH stimulation upregulates NIS expression, which is exploited by withdrawing thyroid hormone replacement or administering recombinant TSH before radioiodine therapy to maximize uptake.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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