What is the function of the thyroid gland?

The thyroid gland controls the metabolism of the body. It produces hormones T3 and T4 which influence how actively every cell in the body functions. Too much thyroid hormone production will cause anxiety, fast heart rate, sometimes weight loss, bowel disturbances, difficulty sleeping and intolerance to hot weather. Too little thyroid hormone causes fatigue, weight gain, hair loss and sensitivity to cold weather.