Can deficiencies in iron, zinc, and selenium worsen autoimmune thyroiditis?

An objective, evidence-based assessment of the relationship between micronutrient deficiencies, thyroid hormone metabolism, and autoimmune thyroiditis is presented below.

Clinical and effectiveness evidence

• Thyroid hormone synthesis and conversion: Deficiencies in iron (Fe), zinc (Zn), and selenium (Se) directly impair the biosynthesis and metabolic conversion of thyroid hormones. Clinical and biochemical studies demonstrate that iron deficiency limits the activity of thyroid peroxidase (TPO)—a heme-containing enzyme crucial for the iodination of thyroglobulin. In randomized and observational trials, correcting iron deficiency anemia in hypothyroid patients significantly improves thyroid hormone profiles.
• Peripheral conversion: Selenium is a mandatory structural component of iodothyronine deiodinases (DIO1, DIO2), the enzymes that convert thyroxine ($T_4$) to active triiodothyronine ($T_3$). Suboptimal selenium levels shift the deiodination pathway, leading to decreased $T_3$ synthesis and an elevated reverse $T_3$ ($rT_3$) ratio. Zinc acts downstream, and its deficiency impairs both $T_3$ production and the structural integrity of the zinc-finger domains in thyroid hormone nuclear receptors, reducing peripheral tissue sensitivity to thyroid hormones.

Mechanistic explanations

• Antioxidant enzyme activity: The thyroid gland is highly metabolically active and generates substantial amounts of hydrogen peroxide ($H_2O_2$) during hormone synthesis. Selenium is an obligate cofactor for glutathione peroxidase (GPx), which detoxifies $H_2O_2$. Zinc is a cofactor for copper/zinc superoxide dismutase (Cu/Zn-SOD), which scavenges superoxide radicals. Deficiencies in these elements lead to a rapid decline in GPx and SOD activities, resulting in accumulated reactive oxygen species (ROS) and elevated lipid peroxidation.
• Autoimmune exacerbation: Oxidative damage to thyrocytes damages cell membranes, causing cell death and exposing intracellular thyroid antigens (specifically TPO and thyroglobulin) to immune cells. This antigenic exposure triggers and sustains a chronic inflammatory response, leading to increased thyroid autoantibody titers (TPOAb and TgAb) and lymphocytic infiltration, which worsens the pathology of autoimmune (Hashimoto's) thyroiditis.

Clinical implications for patients

• Targeted supplementation: Restoring adequate levels of iron, zinc, and selenium under medical supervision can effectively lower oxidative stress markers, restore thyroid enzyme activity, and modestly reduce thyroid autoantibody titers.
• Testing and monitoring: In a 51-year-old female presenting with thyroid dysfunction or autoimmune thyroiditis, routine screening for ferritin, serum zinc, and selenium (or GPx activity) is highly indicated before initiating high-dose supplementation, as excess levels of these micronutrients can also cause toxicity or paradoxically impair thyroid function.

Bottom line

Deficiencies in iron, zinc, and selenium impair key enzymes essential for thyroid hormone production, peripheral activation, and antioxidant defense. The resulting elevation in oxidative stress damages thyroid tissue, exposes intracellular autoantigens, and drives the progression of autoimmune thyroiditis.