Hypothyroidism is a medical condition where your thyroid gland does not produce enough thyroid hormone. Your thyroid is a butterfly-shaped gland located in the front of your throat. An underactive thyroid can disrupt other important bodily functions, such as your heart rate, body temperature, and metabolism. The thyroid gland produces two main hormones called T4 (thyroxine) and T3 (triiodothyronine). When released by the thyroid gland, T4 is mostly converted to T3, the main hormone responsible for biological effects seen in people.