With Hashimoto's thyroiditis, which typically causes hypothyroidism (low thyroid hormone) some patients can have fluctuations from hypothyroid to hyperthyroid and this can be due to having high levels of thyroid antibodies. The antibodies that are tested for, when Hashimoto's is being determined, are the anti-TPO (thyroidperoxidase) and the anti-TG (thyroglobulin) antibodies (either or both testing positive helps confirm it) however, some Hashimoto's patients also test positive for antibodies called the TSI antibodies (thyroid stimulating immunoglobulins). This antibody is what usually contributes to Grave's Disease or "autoimmune hyperthyroidism" however, some Hashimoto's patients have these antibodies as well as the TPO and/or TG ones, that typically cause Hashimoto's and is why they may experience spells of Hashitoxicosis or "intermittent hyperthyroidism". You almost could say they are suffering from Grave's and Hashimoto's, simultaneously.

Even without having the TSI antibodies present, Hashimoto's patients can potentially experience flares of thyroiditis, which can also cause mild hyperthyroid type symptoms that are not as severe as those caused by Hashitoxicosis but are still concerning.

Some patients who have both Hashimoto's and Grave's antibodies that cause continuingly unstable thyroid hormone levels are placed on a treatment called "block and replace". This is where they will block the stimulation of the thyroid with an anti-thyroid medication (slows hormone production) and then replace the patient with thyroid hormone therapy (replaces the diminished hormone levels).

Some Hashimoto's patients have been known to actually transition over to Grave's Disease, when having both types of antibodies and they become progressively hyperthyroid. Other Hashimoto's patients will have hyperthyroid phases but will still become progressively hypothyroid afterward. It may also be encouraging for patients with this condition to know that many Hashimoto's patients have the hyperthyroid spells more-so during the early onset of the disease but after time, the hyperthyroid spells subside and give way to progressive hypothyroidism.