Liothyronine is the most potent form of thyroid hormone. As a salt of triiodothyronine (T 3 ), it is chemically similar and pharmacologically equivalent to T 3 . As such, it acts on the body to increase the basal metabolic rate, affect protein synthesis and increase the body's sensitivity to catecholamines (such as adrenaline) by permissiveness . As monotherapy or in combination therapy with SSRIs , liothyronine may also enhance generation of new neurons in the central nervous system .  The thyroid hormones are essential to proper development and differentiation of all cells of the human body. These hormones also regulate protein, fat, and carbohydrate metabolism, affecting how human cells use energetic compounds.
Treatment of acute massive thyroid hormone overdosage is aimed at reducing gastrointestinal absorption of the drugs and counteracting central and peripheral effects, mainly those of increased sympathetic activity. Vomiting may be induced initially if further gastrointestinal absorption can reasonably be prevented and barring contraindications such as coma, convulsions, or loss of the gagging reflex. Treatment is symptomatic and supportive. Oxygen may be administered and ventilation maintained. Cardiac glycosides may be indicated if congestive heart failure develops. Measures to control fever, hypoglycemia, or fluid loss should be instituted if needed. Antiadrenergic agents, particularly propranolol, have been used advantageously in the treatment of increased sympathetic activity. Propranolol may be administered intravenously at a dosage of 1 to 3 mg, over a 10 minute period or orally, 80 to 160 mg/day, initially, especially when no contraindications exist for its use.
For adult hypothyroidism, levothyroxine is started at -125 mcg/day administered orally. Starting doses and dose changes may differ with individual patients based upon age, the presence of cardiovascular disease , the development of tolerance (reduced effectiveness with continued use), side effects to the medication, and blood levels of thyroid hormone. It may take one to three weeks after initiating therapy with levothyroxine or changing the dose before effects are seen. The goal of replacement therapy is to maintain a normal blood thyroxine level.