In people with Hashimoto thyroiditis, levels of thyroid peroxidase antibodies usually remain elevated. A return to normal levels is uncommon and typically happens only after medical treatments like selenium supplements, thyroid hormone medication, or surgical removal of the thyroid.

In some people with Hashimoto thyroiditis, antibody levels may drop to undetectable levels without surgery, and this change may occur alongside improvement in ongoing hypothyroid symptoms even after taking levothyroxine.

In adults with Hashimoto's thyroiditis who are not taking levothyroxine, taking 200 micrograms of selenium yeast daily for six months is linked to a measurable decrease in two types of thyroid antibodies, while those not taking selenium show no change.

In people with Hashimoto's thyroiditis, lower levels of selenium in the blood are linked to higher levels of antibodies that attack the thyroid gland, even when overall selenium levels are not deficient compared to healthy individuals.

In people with Hashimoto's thyroiditis who have normal selenium levels, taking selenium supplements daily for six months or longer is linked to sustained increases in thyroid antibodies and hormone levels over five years.

A woman with Hashimoto’s thyroiditis, who had been stable on thyroid hormone replacement for ten years, developed hyperthyroidism alongside a change in her autoimmune antibodies from those targeting the thyroid peroxidase to those targeting the thyroid-stimulating hormone receptor.