What is the definition or description of: underactive thyroid?

Too little hormone: The thyroid makes thyroid hormone, the main one is t4, which is converted in other tissues to t3 (liothyronine). An underactive thyroid makes too little thyroid hormone. You feel sluggish, bloated, constipated, cold. It is treated by taking replacement hormone levothyroxine. The thyroid also makes calcitonin, which is involved in calcium metabolism, but it is a weak hormone in humans and not replaced.

Below normal level: Underactive thyroid means a thyroid that isn't able to produce normal levels of thyroid hormone in hour bloodstream. It is easy to check for at your doctor's office and easy to treat to bring you levels back up to normal if indicated.