Regarding the regulation of thyroid hormone synthesis and secretion, all of the following statements are correct EXCEPT:

Explanation

## Thyroid Hormone Synthesis, Secretion, and Regulation ### Correct Statements (Options 0, 1, 2) **Key Point:** TSH acts on the TSH receptor (a G-protein-coupled receptor) on thyroid follicular cells and upregulates the Na⁺/I⁻ symporter (NIS) on the basolateral membrane, increasing iodine uptake — this is the rate-limiting step in thyroid hormone synthesis. **Key Point:** Thyroid peroxidase (TPO) is a heme-containing enzyme that catalyzes two critical reactions: - Iodination of tyrosine residues on thyroglobulin to form monoiodotyrosine (MIT) and diiodotyrosine (DIT) - Coupling of iodotyrosines via peroxidase activity to form T3 (MIT + DIT) and T4 (DIT + DIT) **Key Point:** Both T3 and T4 exert negative feedback on the hypothalamic-pituitary-thyroid (HPT) axis. T4 is preferentially deiodinated to T3 in the anterior pituitary, making T3 the more potent inhibitor of TRH at the hypothalamic level; however, T4 is the dominant feedback inhibitor at the pituitary level. ### Incorrect Statement (Option 3) — THE ANSWER **High-Yield:** Reverse T3 (rT3) is a biologically **inactive** metabolite formed by deamination of T4 at the inner ring (5-deiodinase pathway). It does NOT exert significant physiological effects on target tissues. In contrast, T3 (formed by outer-ring deiodination of T4) is the **active** hormone, 3–5 times more potent than T4 at target tissues. **Clinical Pearl:** During illness, fasting, or stress, conversion of T4 to rT3 increases while conversion to T3 decreases — this is called "low T3 syndrome" and represents an adaptive metabolic downregulation. ### Summary Table: T3 vs T4 vs rT3 | Feature | T4 (Thyroxine) | T3 (Triiodothyronine) | rT3 (Reverse T3) | | --- | --- | --- | --- | | **Source** | Thyroid (80%), peripheral deiodination | Thyroid (20%), peripheral deiodination | Peripheral deiodination of T4 | | **Potency** | Lower (prodrug) | 3–5× higher | Inactive | | **Half-life** | 7 days | 1.5 days | 0.5 days | | **Metabolic effect** | Slow onset, sustained | Rapid onset, brief | None | | **Receptor affinity** | Lower | Higher | Minimal | **Mnemonic:** **rT3 = Rest, Reduce, Recycle** — formed during metabolic stress to conserve energy by reducing active T3 production.