== Information about Thyroid hormone == Thyroid hormones used therapeutically include crude thyroid extracts as well as synthetic forms of L-thyroxine (levothyroxine, T4) and L-triiodothyronine (liothyronine, T3). Thyroid hormone plays an essential role in growth and development and regulates multiple metabolic processes that are responsible for functional homeostasis.
Liver safety of Thyroid hormone
When given in high doses, thyroid hormone preparations can cause mild serum enzyme elevations. In addition, standard doses of levothyroxine have been linked to rare instances of mild, immunoallergic liver injury.
Mechanism of action of Thyroid hormone
Levothyroxine (lee" voe thye rox' een) is an orally available form of T4 that is commonly used to treat hypothyroidism and maintain the euthyroid state. Other forms of thyroid hormone include thyroid extract and triiodothyronine (T3) or liothyronine (lye" oh thye' roe neen). Thyroid hormone is essential for normal growth, particularly of the central nervous system. In adults, thyroid hormone maintains normal metabolism in virtually all organ systems. Thyroxine (T4) is released from the thyroid gland, but is converted in the liver and other tissues to the active form, which is triiodothyronine (T3) which engages thyroid hormone receptors in the nucleus of cells, which together bind to DNA, leading to transcription of thyroid responsive genes that have multiple actions in different cells affecting cell metabolism. Thyroid hormone is necessary for normal growth and development, and deficiency of thyroid hormone results in cretinism in children and hypothyroidism and myxedema in adults with a multitude of symptoms, signs and laboratory abnormalities, including fatigue, weight gain, drowsiness, mental torpor and confusion.
Hypothyroidism is readily treated with oral forms of thyroid hormone which have been in clinical use for over 50 years, the most commonly used being desiccated thyroid (Armour Thyroid, 1950s), synthetic L-thyroxine or levothyroxine (T4, Synthroid, Levoxyl and others: 2002), and L-triiodothyronine or liothyronine (T3, Cytomel: 1956). Levothyroxine is currently one of the most commonly prescribed medications in the United States, with more than 100 million prescriptions filled yearly.
Clinical use of Thyroid hormone
The current indications are for maintenance of the euthyroid state.
Dosage and administration for Thyroid hormone
Levothyroxine is available in tablets at of 25 to 300 mcg for oral administration and as a lyophilized powder for parenteral use. Levothyroxine is typically started at a low daily dose (25 to 50 mcg) and increased based upon clinical effect and serum levels of thyroid stimulating hormone (TSH) and free T4. The usual adult replacement dose is 75 to 125 mcg daily.
Side effects of Thyroid hormone
Side effects are uncommon at correct replacement doses, but high doses can cause symptoms of hyperthyroidism such as fatigue, weight loss, headache, anxiety, tremor, muscle weakness, tachycardia, cardiac arrhythmias, menstrual abnormalities, irritability, emotional labiality, sleep disturbance and changes in personality.
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