Minerals and Thyroid Health: Iodine, Selenium, Zinc and Iron Together

Each mineral and the thyroid

Iodine

The NHS (UK, page reviewed 2020) says adults need 140 micrograms (mcg) of iodine a day and that most people should get enough from a varied and balanced diet. [5] It advises anyone considering a supplement to seek advice from a healthcare professional. [5] It says taking high doses for long periods could change the way the thyroid works. [5]

A review in the Annals of the New York Academy of Sciences describes how excess iodine may bring on overactive thyroid, underactive thyroid, goiter or thyroid autoimmunity. [2] People with existing thyroid disease, or those previously low in iodine, may be more susceptible, and iodine-induced overactive thyroid can be life-threatening in some individuals. [2] Excess iodine can come from certain seaweeds and iodine-containing supplements. [2] See kelp and seaweed supplements and iodine and the thyroid.

Selenium

Selenium and Hashimoto thyroiditis is the best-studied question; the 2024 review describes it as the most common cause of underactive thyroid in iodine-sufficient areas. [3] A 2024 meta-analysis of randomized trials included 35 studies and judged the certainty moderate. [3] In people not on thyroid hormone replacement, selenium lowered TSH; thyroid antibodies (TPOAb) and a marker called MDA fell with or without replacement. Adverse effects were comparable to control. [3] Thyroid hormone levels such as free T4 and T3 did not significantly change. [3] Lower antibodies are a lab result, not proof of feeling better.

An earlier Cochrane summary found only four studies at unclear or high risk of bias, and said their clinical relevance was unclear. [6] It judged the evidence incomplete and not reliable. [6] We label this promising but limited. For Graves disease, the 2021 European guideline for eye involvement says that in mild, active disease selenium in selenium-deficient areas is usually sufficient alongside other measures, which is a clinician-led decision. [7] More here: selenium and the thyroid.

The NHS notes too much selenium causes selenosis, which in its mildest form can lead to loss of hair and nails. [8] The reviewers also warn that where iodine and selenium are both very low, iodine supply should be normalized before selenium is started. [1]

Iron and zinc

Iron-deficiency anemia blunts, and iron supplementation improves, the effectiveness of iodine supplementation. [1] That is a reason to ask a clinician about iron testing, not to take iron untested. Very high doses of iron can be fatal, particularly to children, so keep iron out of reach. [9]

In a randomized trial of 68 hypothyroid women, zinc plus selenium for 12 weeks (30 mg zinc as zinc gluconate and 200 mcg selenium as selenium yeast, a study amount) produced no significant treatment effects between groups for TSH, free T4, total T3 or total T4. [4] Some within-group changes were seen (for example, TSH fell in the zinc-plus-selenium group), but not differences between groups. [4] It is a small, single trial.

Official levels (UK, adults 19 to 64, NHS, 2020)

US and EU figures differ, and we could not read US National Institutes of Health fact sheets this session.