The thyroid hormone, thyroxine (T4) is a tyrosine-based hormone which is produced by the thyroid gland. Synthetic versions of this hormone are used to make up for a lack of naturally produced thyroxine and increase the rate of metabolism. Artificially made thyroid hormone improves the symptoms of hypothyroidism (underactive thyroid) and speeds the rate of metabolism in those who suffer from the disease. Thyroid hormones may also be used to treat goiter... -- more about T4 --