The sodium-glucose cotransporter in the thyroid follicular cell (NIS — sodium iodide symporter) is the primary mechanism for iodide accumulation in the thyroid. The NIS establishes an iodide concentration gradient of approximately 25–50× plasma. Its activity depends on which electrochemical driving force?
- A Inward Na⁺ gradient maintained by Na-K-ATPase on the basolateral membrane; NIS cotransports 2 Na⁺ with 1 I⁻ inward against the I⁻ electrochemical gradient ✓
- B Outward K⁺ gradient maintained by K⁺ channels; NIS uses K⁺ efflux to drive I⁻ uptake
- C Active H⁺ pump on the apical membrane generating proton motive force for I⁻ uptake
- D Cl⁻ gradient maintained by CFTR channels; NIS exchanges Cl⁻ for I⁻ electrically
Correct answer: A. Inward Na⁺ gradient maintained by Na-K-ATPase on the basolateral membrane; NIS cotransports 2 Na⁺ with 1 I⁻ inward against the I⁻ electrochemical gradient
Explanation
NIS is a secondary active transporter on the basolateral membrane of thyroid follicular cells that cotransports 2 Na⁺ ions with 1 I⁻ ion into the cell, using the inward electrochemical Na⁺ gradient (maintained by basolateral Na-K-ATPase) as its driving force. The net electrogenic inward movement of 2 positive charges with each transport cycle also makes NIS voltage-dependent. TSH upregulates NIS expression, and perchlorate/thiocyanate competitively inhibit it. On the apical side, pendrin (and TMEM16A/anoctamin 1) facilitates I⁻ efflux into the follicular lumen.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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