Thyroid health and diet: iodine, selenium, and the Hashimoto's question

Few topics generate as many confident claims in my clinic as the thyroid. Patients arrive with kelp tablets, high-dose iodine drops and selenium capsules, hoping to fix fatigue or autoimmune thyroid disease through supplements alone. My job is to separate the genuinely useful from the hopeful, and to be honest about where the evidence is thin.

Thyroid 101: a small gland with a big job

Your thyroid produces the hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolism, body temperature, mood and energy. The pituitary controls output via thyroid-stimulating hormone (TSH), which is why a blood TSH is usually the first test ordered. An underactive thyroid causes fatigue, weight gain and cold intolerance; an overactive one the opposite. In iodine-sufficient countries like Australia, the most common cause of hypothyroidism is Hashimoto's thyroiditis, an autoimmune condition often marked by raised thyroid peroxidase antibodies.

Iodine: essential, but the dose matters enormously

Iodine is the non-negotiable raw material for thyroid hormone. Globally, iodine deficiency remains the leading preventable cause of impaired brain development, which is why salt iodisation has been one of the most effective public-health measures of the last century.^[1] Here is the part that surprises people: the relationship between iodine and thyroid health is U-shaped. Both too little and too much iodine raise risk. A large analysis of studies covering nearly 70,000 adults found that both iodine deficiency and iodine excess increased the risk of thyroid autoimmunity, compared with a more-than-adequate intake.^[2] A separate systematic review of populations exposed to excess iodine found higher rates of hypothyroidism and autoimmune thyroiditis.^[3] The WHO recommends about 150 micrograms of iodine per day for non-pregnant adults, rising in pregnancy.^[1] Some high-dose supplements provide hundreds of times that — a pharmacological dose that does not belong in self-directed care.

Where to get iodine from food

Meeting your iodine needs through food is straightforward for most people: iodised salt, dairy, eggs and seafood. In Australia, mandatory iodine fortification of bread has improved population iodine status. Seaweed is genuinely rich in iodine, but its content is wildly variable and kelp supplements can deliver excessive amounts, so I treat them with caution rather than enthusiasm.

Selenium and Hashimoto's: measured, not magical

Selenium is a trace mineral the thyroid uses in the enzymes that activate thyroid hormone and protect the gland. An early, influential meta-analysis found that selenium supplementation (typically 200 micrograms daily) significantly reduced thyroid antibody levels over 6 to 12 months in autoimmune thyroiditis.^[4] But a rigorous Cochrane review reached a more sober conclusion: the trials were small and at unclear-to-high risk of bias, and the evidence was "incomplete and unreliable" for guiding decisions.^[5] A fall in an antibody number is not the same as feeling better or preventing progression. I view selenium as plausible but unproven — and it has a narrow safe range, so more is not better.

Where to get selenium from food

Brazil nuts are the standout source — just one or two per day can meet the adult requirement, which is also why a large handful daily is too much. Other good sources include fish and seafood, eggs, poultry, meat and wholegrains.

What not to do

If you take one message from this article: do not self-prescribe high-dose iodine or selenium. Megadosing iodine can trigger or worsen thyroid dysfunction in susceptible people,^[2][3] and high-dose selenium carries its own toxicity. Thyroid symptoms are non-specific, so a real diagnosis requires blood tests and ongoing medical review.

This article is general information and not a substitute for personalised medical advice. If you have a thyroid condition or suspect one, please speak with your doctor.

References

1. Zimmermann MB. Iodine-deficiency disorders. Lancet. 2008. PubMed
2. Wang B, et al. U-shaped relationship between iodine status and thyroid autoimmunity risk in adults. Eur J Endocrinol. 2019. PubMed
3. Katagiri R, et al. Effect of excess iodine intake on thyroid diseases in different populations: a systematic review and meta-analyses including observational studies. PLoS One. 2017. PubMed
4. Toulis KA, et al. Selenium supplementation in the treatment of Hashimoto's thyroiditis: a systematic review and a meta-analysis. Thyroid. 2010. PubMed
5. van Zuuren EJ, et al. Selenium supplementation for Hashimoto's thyroiditis: summary of a Cochrane systematic review. Eur Thyroid J. 2014. PubMed