Selenium and Hashimoto's thyroiditis: what UK sources say about supplements

Hashimoto's thyroiditis (also called Hashimoto's disease) is an autoimmune condition. The NHS says that in the UK it is the main cause of an underactive thyroid, because the immune system attacks the thyroid gland so that it cannot make enough thyroid hormone. It is most common in women and usually starts between the ages of 30 and 50. Symptoms develop slowly and include tiredness, feeling cold, weight gain, constipation, difficulty concentrating, low mood, dry skin and hair, and a croaky voice.

The British Thyroid Foundation (BTF) explains in its supplements guide that selenium is essential in converting the thyroid hormone T4 into T3, the active form of thyroid hormone. That biological role is the origin of interest in selenium supplements for Hashimoto's, and it is why many people ask whether it can reduce antibody levels or slow the disease.

The BTF guide to diets and supplements for thyroid disorders discusses selenium mainly in two situations. First, it says the evidence of benefit is mainly in people with mild thyroid eye disease, for which selenium is recommended, and describes a large European study in which six months of selenium supplements had a beneficial effect on thyroid eye disease and were associated with improved quality of life. Second, it says some trials show selenium can bring thyroid antibody levels down in patients with Graves' disease, but that there does not appear to be any link between taking selenium and improvement in thyroid function.

The BTF page does not set out a specific recommendation for selenium in Hashimoto's thyroiditis, and neither does the NHS page on underactive thyroid. Trials specific to Hashimoto's are not summarised in the UK patient sources used for this page, and a change in antibody levels is not the same as feeling better or needing less levothyroxine. If you are considering selenium, ask your GP or endocrinologist how it relates to your own test results.

The NHS vitamins and minerals guide says selenium helps the immune system work properly and in reproduction. Good sources are Brazil nuts, fish, meat and eggs, and the amount needed is 75 micrograms a day for men and 60 micrograms a day for women aged 19 to 64. If you eat meat, fish or nuts you should be able to get all you need from your diet. The BTF lists Brazil nuts, tuna, sardines, eggs and legumes.

The NHS says too much selenium causes selenosis, which in its mildest form can lead to loss of hair and nails, and that taking 350 micrograms or less a day of supplements is unlikely to cause harm. The BTF gives a warning that exceeding the recommended intake can often do more harm than good and cites a study showing that high-dose selenium supplements (more than 300 micrograms a day) increased all-cause mortality. Check the label of any product, and remember that multivitamins may also contain selenium.

The BTF advises that, to avoid absorption problems, and unless otherwise indicated, supplements should be taken at least four hours apart from levothyroxine. It also warns that some calcium-rich foods and supplements interfere with levothyroxine absorption and that soya interferes with it. If you take levothyroxine for an underactive thyroid there is no need to take iodine supplements, according to the BTF.

Do not stop or reduce levothyroxine because you have started a supplement. The NHS says that being on the wrong dose can cause side effects, and that if untreated an underactive thyroid can increase the risk of heart and bone problems. Dose changes need blood tests and a clinician's advice.

For most people with Hashimoto's, the key steps are the correct dose of levothyroxine, regular blood tests and attention to symptoms. The NHS says that if you have an underactive thyroid and are pregnant or planning pregnancy you should tell your GP, because the levothyroxine dose is likely to need to change. The NHS says a varied and balanced diet should provide all the selenium you need.

The British Thyroid Foundation and Thyroid UK both provide support and information for people living with thyroid conditions, and the NHS page signposts both.

Thyroid function tests, antibody tests and levothyroxine prescribing are available through the NHS. Private thyroid panels and supplement advice are widely marketed; if you use them, share the results with your GP so that levothyroxine decisions are made on a full picture, and be wary of any product claiming to reverse Hashimoto's.