A 50-year-old woman with well-differentiated papillary thyroid carcinoma undergoes radioiodine (I-131) therapy post-thyroidectomy. The mechanism by which differentiated thyroid cancer cells take up radioiodine is via:
- A Passive diffusion of iodine across cell membranes
- B Thyroid peroxidase oxidation of iodide at the apical membrane
- C Sodium-iodide symporter (NIS) expressed on follicular epithelial cells, driven by the sodium gradient ✓
- D Thyroglobulin receptor-mediated endocytosis
Explanation
The sodium-iodide symporter (NIS) is a plasma membrane glycoprotein on thyroid follicular cells that co-transports 2 Na+ and 1 I- into the cell, driven by the basolateral Na-K-ATPase sodium gradient. Differentiated thyroid carcinoma cells (papillary and follicular) retain NIS expression to variable degrees, allowing I-131 uptake for ablation of residual thyroid tissue or metastases. TSH stimulation (endogenous or recombinant) upregulates NIS expression, improving I-131 uptake. Thyroid peroxidase mediates iodide oxidation and organification (coupling to thyroglobulin) but is a separate step after NIS-mediated uptake.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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