Thyroxine is a tetra-iodinated tyrosine molecule. It is an important hormone that is secreted by the thyroid gland, which is a butteryfly shaped gland overlying the upper trachea, just below the larynx. This hormone, together with tri-iodo-thyronine (same as the thyroxine molecule minus one of the iodine atoms), controls the overall metabolism of the body. The secretion of thyroxine and tri-iodo-thyronine is controlled by the hypothalamus and the anterior pituitary gland, the latter secreting thyroid stimulating hormone (commonly refered to as TSH), which stimulates the thyroid tissue to produce more thyroxine and tri-iodo-thyronine. As these levels increase, there is a negative feedback on the hypothalamus and pituitary gland to keep the secretion of TSH in check.
Underactivity of the thyroid gland causes low thyroxine and tri-iodo-thyronine levels in the blood resulting in slowing of the metabolism and a medical condition known as myxoedoema. Overactivity of the thyroid gland produces high thyroxine and tri-iodo-thyronine levels in the blood, causing overactivity of the metabolism, resulting in weightloss, a condition known as thyrotoxicosis. Both of these are common endocrine conditions, particularly in women.