To help diagnose hyperthyroidism. Hyperthyroidism is a condition in which the thyroid gland is overactive, and makes too much thyroid hormone. The thyroid gland is located in the neck. It produces several hormones that control the way every cell in the body uses energy (metabolism). The thyroid is part of the endocrine system.Triiodothyronine (T3) is a thyroid hormone. It plays an important role in the body's control of metabolism.