Selenium

The thyroid contains more selenium than any other organ, and selenium-dependent enzymes help make active thyroid hormone. Some trials in Hashimoto's thyroiditis found that selenium lowered thyroid antibody levels, but reviews conclude it is unclear whether this improves symptoms or outcomes, and it does not replace thyroid medication. Selenium is toxic at high doses.

People with thyroid conditions should ask their endocrinologist before using selenium. Whether it suits you depends on your health, medicines and test results — ask a clinician.

Trials have commonly used about 100–200 mcg daily. The NIH upper limit is 400 mcg per day from all sources. This is not a recommendation. Doses vary by product and person; this is not personal medical advice.

Studies measured antibodies after about 3–6 months.

Excess selenium can cause garlic breath, metallic taste, hair or nail changes, stomach upset and nerve effects.

Do not exceed 400 mcg per day from all sources, including Brazil nuts and multivitamins. Do not stop thyroid medicine without medical advice.

Evidence level (our editorial summary, not a formal grading): Limited to moderate (antibody markers); clinical benefit unclear.

Pregnancy & breastfeeding: Selenium is part of normal prenatal nutrition; do not exceed the upper limit; ask your provider before taking extra.

Medication interactions: May interact with some medicines, including certain chemotherapy drugs and blood thinners; ask a pharmacist.

Talk to a healthcare professional before use if you are pregnant, breastfeeding, planning pregnancy, have a medical condition, take any medicine, or are scheduled for surgery. Dietary supplements are not approved by the FDA for safety or effectiveness before they are sold, and quality varies between products.

Reliable sources to check: NIH Office of Dietary Supplements fact sheets · NCCIH Herbs at a Glance · MedlinePlus Herbs & Supplements · FDA — Dietary Supplements