Hyperthyroidism is the term for overactive tissue within the thyroid gland that causes an overproduction of thyroid hormones (thyroxine or "T4" and/or triiodothyronine or "T3"). Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous. For instance, thyrotoxicosis could instead be caused by ingestion of exogenous thyroid hormone or inflammation of the thyroid gland, causing it to release its stores of thyroid hormones. Its opposite is hypothyroidism which is an underproduction of the hormone.