The thyroid is a butterfly-shaped gland that sits in the front of the neck. It produces hormones that are essential for heart rate, digestion, mood, and other functions crucial to human health.

Thyroid level tests, including thyroid-stimulating hormone (TSH) and free T4, help diagnose underactive or overactive thyroid. Doctors may also check free T3, calcitonin, thyroglobulin, or thyroid ...