How often should get tested for hyperthyroidism (overactive thyroid)?

Biannually: Depending on your med hx, but at least every 6 mths to annually to monitor TSH levels and adjust any medications you're on.

Depends on condition: It depends on what is going on with your thyroid. If there is no reason to suspect hyperthyroidism, a tsh/ft4 is not usually checked. If you have graves disease and are on antithyroid meds (eg methimazole), then you need to be checked every 3-6 months, depending on symptoms. With thyroiditis (different kinds), even more frequent checks may be needed. See your endocrinologist.