A blood test measures the levels of thyroid hormone (thyroxine, or T4) and thyroid-stimulating hormone (TSH) in your blood. Your pituitary gland releases TSH to stimulate your thyroid to produce its hormones. High thyroxine and low TSH levels can indicate that your thyroid gland is overactive.

A doctor might also give you radioactive iodine by mouth or as an injection and then measure how much of it your thyroid gland takes up. Your thyroid takes in iodine to produce its hormones. Taking in a lot of radioactive iodine is a sign that your thyroid is overactive. This low level of radioactivity resolves quickly and isn’t dangerous for most people.

Treatments for hyperthyroidism destroy your thyroid gland or block it from producing its hormones. Treatments can include:

Antithyroid medication. Antithyroid drugs such as methimazole (Tapazole) can prevent your thyroid from producing its hormones and reduce symptoms.

Radioiodine therapy. A large dose of radioactive iodine damages your thyroid gland. You can take it as a pill or liquid by mouth. As your thyroid gland takes in iodine, it also pulls in the radioactive iodine, which damages the gland. But this option isn’t safe for people who are pregnant or breastfeeding.

Beta-blockers. Beta-blockers can help lessen symptoms.

Surgery. Surgery can be performed to remove your thyroid gland. This may return your thyroid levels to typical ones.

If you have radioactive iodine treatment or surgery that destroys your thyroid gland, you’ll develop hypothyroidism and need to take thyroid hormone daily.