The major function of the thyroid gland is to produce Thyroid hormones. The two main thyroid hormones are thyroxin (T4) and triiodothyronine (T3). These are excreted into the blood stream and regulate metabolism in the rest of the body. Thyroid hormone plays a role in everything from hair production to heart function. The thyroid gland in turn is regulated by thyroid stimulation hormone (TSH) produced by the pituitary gland. TSH, T4, T3, and Thyrotrophic releasing hormone all act in a feedback loop controlled by the hypothalamus and pituitary gland to regulate the amount of thyroid hormone in the body. When the thyroid gland is removed surgically or when the function of the gland is impaired from medical conditions, hypothyroidism results, and hormone replacement is necessary. Fortunately, thyroid hormone replacement can be taken by mouth, and levels of TSH can be followed in order to ensure adequate replacement. When there is excessive thyroid hormone production from a tumor (hot nodule) or from autoimmune disorder like Grave’s disease, hyperthyroidism results.