Your thyroid produces hormones that affect almost every cell in your body. Thyroid hormones thyroxine (T4) and triiodothyronine (T3) help regulate the rate your body uses energy. Thyroid-stimulating hormone (TSH) is produced in the pituitary gland and regulates the production of T3 and T4. It is usually measured as a proxy for thyroid hormones T4 and T3. When circulating hormones are low, TSH elevates, and when circulating hormones are high, TSH will be low. Raised levels of TSH can indicate that the thyroid is struggling to produce sufficient quantities of thyroid hormones and is underactive. Low levels can show that the thyroid produces excess levels of thyroid hormones and is overactive.