Thyroid function tests are a series of blood tests used to measure how well your thyroid gland is working. Available tests include the T3, T3RU, T4, and TSH. The thyroid is a small gland located in the lower-front part of your neck. It’s responsible for helping regulate many bodily processes, such as metabolism, energy generation, and mood. The thyroid produces two major hormones: triiodothyronine (T3) and thyroxine (T4). If your thyroid gland doesn’t produce enough of these hormones, you may experience symptoms such as weight gain, lack of energy, and depression. This condition is called hypothyroidism. If your thyroid gland produces too many hormones, you may experience weight loss, high levels of anxiety, tremors, and a sense of being on a “high.” This is called hyperthyroidism.

Typically, a doctor who is concerned about your thyroid hormone levels will order broad screening tests, such as the T4 or the thyroid-stimulating hormone (TSH) test. If those results come back abnormal, your doctor will order further tests to pinpoint the reason for the problem.