How do selenium, iron, and zinc affect thyroid hormones?

Thyroid physiology depends on several micronutrient-containing enzymes, but a biochemical requirement does not necessarily mean that supplementation will improve thyroid tests in an otherwise replete person.

Clinical and biochemical evidence

• Selenium: DIO1 and DIO2 are selenoenzymes containing catalytic selenocysteine and catalyze outer-ring deiodination of T4 to active T3. DIO1 contributes to circulating T3, while DIO2 generates T3 locally within tissues. This mechanism is firmly established.
• Iron: Iron is required for thyroid peroxidase activity, a key step in thyroid-hormone synthesis. Meta-analytic observational evidence associates iron deficiency with lower free T4 and free T3, but findings are highly heterogeneous; pregnancy studies vary according to whether deficiency is defined by ferritin or hemoglobin.
• Zinc: Zinc has credible roles in thyroid-hormone receptor structure and transcriptional regulation, but its effects on synthesis or deiodinase-mediated T4-to-T3 conversion are not established. In a small randomized trial of levothyroxine-treated hypothyroid women, 30 mg/day for 12 weeks increased free T3 and the free-T3/free-T4 ratio, but broader supplementation evidence is inconclusive.

Mechanistic context

• Selenium is also required for synthesis of DIO1, DIO2, and DIO3. Whereas DIO1/DIO2 activate T4 to T3, DIO3 inactivates T4 through inner-ring deiodination to reverse T3.
• Thus, micronutrient status could affect hormone production or tissue-specific activation, but serum T3/T4 reflect many additional regulatory processes.

Clinical implications

• Selenium trials have not consistently increased T3 or conversion ratios, including in selenium-deficient adults; Hashimoto thyroiditis meta-analysis found no pooled free-T3 or free-T4 benefit.
• Correct documented iron, selenium, or zinc deficiency for general health, but do not use supplements as substitutes for thyroid evaluation or prescribed therapy.

Bottom line

• The claim is substantially correct: selenium-dependent T4-to-T3 activation and iron-dependent hormone synthesis are established; zinc’s contribution is plausible but less certain, and clinically meaningful thyroid-test improvement with supplementation is not reliably demonstrated.