When a person with long-term Hashimoto's thyroiditis develops symptoms of an overactive thyroid, doctors must rule out excessive levothyroxine dosing and other non-autoimmune causes before diagnosing Graves' disease, because the signs and lab results can look very similar.

Although autoimmune thyroid diseases like Hashimoto's and Graves' disease are common, switching from one to the other is very rare, with fewer than 50 confirmed cases reported in medical records.

Sometimes people can have Graves’ disease and show symptoms like bulging eyes or skin issues even if their thyroid isn’t overactive—this can happen if their thyroid doesn’t work well enough to produce excess hormones, and it means doctors might miss the disease if they only check thyroid levels.

In people with early-stage Hashimoto’s thyroiditis who are not taking thyroid medication, taking 200 micrograms of selenium yeast daily for six months is linked to a small decrease in TSH hormone levels, whereas those not taking selenium experience a small increase.

If someone has an underactive thyroid and signs of an overactive immune system — especially bulging eyes — doctors should still think about Graves’ disease, even if the thyroid isn’t overactive.

Some people with certain types of thyroid inflammation (like Hashimoto's or subacute thyroiditis) can have a substance called TDA in their blood, but it doesn't actually turn on the thyroid like it does in Graves’ disease — so TDA might not mean the thyroid is being overstimulated.