Selenium and Thyroid Health

Selenium is a trace mineral that rarely makes headlines, yet it plays a quietly important role in keeping the body running smoothly. You need only a small amount, but that amount matters: selenium is built into a family of proteins that act as antioxidants and help regulate two systems in particular — the thyroid and the immune system.

What selenium does in the body

Selenium works mainly as part of enzymes called selenoproteins. These act as antioxidants, helping to protect cells from the kind of oxidative stress that occurs as a normal by-product of everyday metabolism. It also contributes to a healthy immune response and to the way the body handles thyroid hormones.

The thyroid connection

The thyroid gland contains one of the highest concentrations of selenium in the body, and for good reason. Selenium-dependent enzymes help convert the thyroid's main hormone into its more active form and protect the gland from oxidative damage during hormone production. Because of this close relationship, selenium status is sometimes considered when looking at thyroid health, particularly in autoimmune thyroid conditions. Any decisions about supplementation, however, should be made with a clinician rather than assumed.

Too little and too much

Selenium is unusual in that both deficiency and excess can cause problems, and the safe range is relatively narrow. Low selenium is uncommon in the UK but can occur with certain diets, digestive conditions affecting absorption, or in some parts of the world where soil levels are low. At the other end, taking high-dose supplements over time can lead to excess, sometimes called selenosis, with symptoms such as:

• Hair and nail changes or brittleness
• A garlic-like odour on the breath
• Digestive upset
• Fatigue or irritability

This narrow window is exactly why a blood test is more sensible than guessing before topping up.

How selenium is tested

A red cell selenium test measures the selenium held within your red blood cells, which reflects your status over a longer period than a single serum snapshot. This makes it a useful way to check whether your level sits comfortably within the healthy range, whether you are considering supplements, or whether you have symptoms or risk factors that raise the question.

Who might consider testing

Testing can be worthwhile if you already take selenium supplements and want to be sure you are not overdoing it, if you have an autoimmune thyroid condition, if you follow a restricted diet, or if you have a digestive condition affecting absorption. As always, results are best interpreted alongside your wider health and any thyroid findings by a qualified clinician.

Written by Dr Ibrar Ahmed — MBChB MRCP MRCGP PhD PG Dip (Cardiology)
Specialist interest: Cardiovascular Medicine · OneMedicine, CQC-registered clinic