Ten to fifteen percent of the population have positive thyroid antibodies, most commonly thyroid peroxidase (TPO) antibodies, with a higher prevalence in women than men. The thyroid is a very complex organ. Iodine is an indispensable component of thyroid hormones, comprising 65% of T4’s weight, and 58% of T3’s weight.1

Clinically, iodine in relation to thyroid health is a controversial topic. Iodine deficiency is the primary reason for hypothyroidism worldwide. Thus, public health authorities in many countries have decided to add iodine to bread and salt. However, iodine has a very narrow therapeutic index, so this action has come with a consequence. On one hand, iodine deficiency can lead to hypothyroidism due to a lack of building blocks for thyroid hormones.  On the other hand, an excess of it can lead to a higher risk of developing autoimmune thyroid disease.2

The American Thyroid Association and Australian National Health and Medical Research Council caution against using doses of more than 500μg per day in the general public and noted that doses above 1100μg may cause thyroid dysfunction. These warnings are for the general population, but studies have found that people with Hashimoto’s may be sensitive to even smaller doses.3

When it comes to selenium supplementation, a study measured the impact of long-term (9 months) supplementation with two doses of selenomethionine (100/200μg per day) on autoimmune thyroiditis, particularly on the concentration of TPO and thyroglobulin antibodies. The authors found that the group taking 200μg/day had a sharp decrease in serum levels of TPO antibodies until 6 months of treatment, after which the values tended to level off (26.6% at 3 months, 26.2% at 6 months, and 3.6% at 9 months). In contrast, the group of patients supplemented with 100μg/day showed an increase of 38.1% in the level of TPO antibodies. However, when this same group of patients received the higher amount (200μg/day), there was a decrease of 30.3% in the level of TPO antibodies. The authors concluded that the oral administration of 200μg/day of selenomethionine effectively reduces serum levels of TPO antibodies.4

All references available upon request.