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.

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.

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.

Sometimes people with Graves’ disease get eye or skin problems even when their thyroid hormone levels are normal, which suggests these issues might be caused by the immune system attacking tissues directly, not just from too much thyroid hormone.

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.

Some people with Graves disease, an autoimmune condition that makes the thyroid overproduce hormones, can suddenly stop needing medication because their thyroid starts working normally again—even though their immune system is still attacking it.