Can both low and high iodine intake influence autoimmune thyroiditis?

The relationship between dietary iodine intake and thyroid health is characterized by a non-linear, U-shaped curve. Both iodine deficiency and iodine excess can influence the development and progression of autoimmune thyroiditis, with higher iodine intake specifically driving thyroid autoantibody positivity in genetically and environmentally susceptible individuals.

Clinical and epidemiological evidence

Epidemiological studies and clinical trials demonstrate that deviation from optimal iodine levels in either direction increases the risk of thyroid autoimmunity.

• Iodine Deficiency: In cohorts of pregnant women and children, iodine deficiency is strongly associated with increased thyroid autoantibody positivity. For instance, studies have shown that non-iodized salt use or low urinary iodine concentration (UIC < 100 μg/L) correlates with elevated thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb) levels.
• Iodine Excess: Chronic exposure to high iodine levels (UIC > 300 μg/L) is robustly linked to a rise in thyroid autoantibody positivity. Population studies consistently demonstrate that individuals residing in regions with high-iodine water supplies or those undergoing rapid dietary iodine fortification have significantly higher odds ratios for TPOAb and TgAb positivity.

Mechanistic explanations

The pathological effects of dietary iodine on the thyroid gland are mediated by distinct biochemical and immunological pathways:

• Enhanced Antigenicity via Thyroglobulin Iodination: Higher iodine intake directly leads to increased iodination of thyroglobulin (Tg) within the thyroid colloid. This heavy iodination alters the physical conformation of the Tg protein, exposing hidden antigenic determinants and creating highly immunogenic neo-epitopes. These newly formed epitopes stimulate T-cell and B-cell recognition, driving the production of TgAb.
• Intrathyroidal Oxidative Stress and Apoptosis: Excessive iodine accelerates thyroid peroxidase activity and hydrogen peroxide ($H_2O_2$) generation. This process increases intrathyroidal oxidative stress, overwhelming local antioxidant defenses. The resulting accumulation of reactive oxygen species (ROS) triggers thyrocyte apoptosis, leading to cell death and the release of sequestered intracellular self-antigens (such as TPO and Tg) into the inflammatory microenvironment.

Clinical implications and susceptibility

The effect of high iodine intake on autoantibody positivity is not uniform; it is highly qualified by individual susceptibility:

• Baseline Status and Co-factors: Individuals transitioning rapidly from a baseline state of iodine deficiency to sufficiency or excess are at the highest risk for acute autoimmune activation.
• Role of Selenium: Selenium acts as an essential co-factor for glutathione peroxidases, which neutralize the excess $H_2O_2$ generated during iodine processing. Clinical evidence shows that adequate selenium status can mitigate or entirely eliminate the risk of iodine-induced thyroid autoantibody positivity by suppressing oxidative stress and thyrocyte injury.

Bottom line

Both iodine deficiency and excess modulate thyroid autoimmunity. To minimize the risk of triggering or exacerbating autoimmune thyroiditis, susceptible individuals should aim for a stable, optimal iodine intake while avoiding extreme deficiency or sudden, high-dose iodine supplementation, especially when protective trace elements like selenium are suboptimal.