In older adults with low selenium levels, taking selenium and coenzyme Q10 supplements for four years results in a smaller rise in thyroid-stimulating hormone (TSH) levels compared to taking a placebo, and fewer people reach the TSH level that defines subclinical hypothyroidism.

Taking 200 micrograms of selenium daily for a year raises selenium levels in the blood of adults with autoimmune hypothyroidism who are on levothyroxine, but it does not change how efficiently the body converts thyroxine into triiodothyronine.

Taking 200 micrograms of selenium daily for a year does not improve how people with autoimmune hypothyroidism feel on a standardized quality-of-life survey, even though their thyroid antibody levels decrease.

In adults with autoimmune hypothyroidism, quality of life improved over 12 months whether they took selenium or a placebo, suggesting the improvement was not due to selenium but possibly to other factors like expectation or natural changes in the condition.

In older adults with low selenium levels, taking 200 micrograms of selenium and 200 milligrams of coenzyme Q10 daily for four years is associated with an increase in free triiodothyronine and a decrease in free thyroxine, indicating a change in thyroid hormone conversion that may influence metabolic and cardiovascular health.

In people with autoimmune hypothyroidism taking levothyroxine, taking 200 micrograms of selenium daily for a year may lower levels of thyroid peroxidase antibodies by about 19% compared to a placebo, but it does not change the required dose of thyroid hormone or the balance between free triiodothyronine and free thyroxine.