Levothyroxine, also known as L-thyroxine, is a manufactured form of the thyroid hormone thyroxine (T4). It is used to treat thyroid hormone deficiency (hypothyroidism), including and a severe form known as myxedema coma. It may also be used to treat and prevent certain types of thyroid tumors. It is not indicated for weight loss. Levothyroxine is taken by mouth or given by intravenous injection. Maximum effect from a specific dose can take up to six weeks to occur.Side effects from excessive doses include weight loss, trouble tolerating heat, sweating, anxiety, trouble sleeping, tremor, and fast heart rate. Use is not recommended in people who have had a recent heart attack. Use during pregnancy has been found to be safe. Dosing should be based on regular measurements of thyroid-stimulating hormone (TSH) and T4 levels in the blood. Much of the effect of levothyroxine is following its conversion to triiodothyronine (T3).Levothyroxine was first made in 1927. It is on the World Health Organizations List of Essential Medicines. Levothyroxine is available as a generic medication. In 2020, it was the second most commonly prescribed medication in the United States, with more than 98 million prescriptions. Medical use
Levothyroxine is typically used to treat hypothyroidism, and is the treatment of choice for people with hypothyroidism who often require lifelong thyroid hormone therapy.It may also be used to treat goiter via its ability to lower thyroid-stimulating hormone (TSH), which is considered goiter-inducing.
A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran. Certain other substances can cause adverse effects that may be severe. Combination of levothyroxine with ketamine may cause hypertension and tachycardia; and tricyclic and tetracyclic antidepressants increase its toxicity. Lithium, though, can cause hyperthyroidism (but most often hypothyroidism)  by affecting iodine metabolism of the thyroid itself, thus inhibiting synthetic levothyroxine, as well. Soy,  walnuts, fiber, calcium supplements, and iron supplements can also adversely affect absorption. A study found that cows milk reduces levothyroxine absorption.To minimize interactions, a manufacturer of levothyroxine recommends after taking it,  waiting 30 minutes to one hour before eating or drinking anything that is not water. They further recommend to take it in the morning on an empty stomach. Mechanism of action
Levothyroxine is a synthetic form of thyroxine (T4), an endogenous hormone secreted by the thyroid gland, which is converted to its active metabolite, L-triiodothyronine (T3). T4 and T3 bind to thyroid receptor proteins in the cell nucleus and cause metabolic effects through the control of DNA transcription and protein synthesis. Like its naturally secreted counterpart, levothyroxine is a chiral compound in the L-form. Pharmacokinetics
Absorption of orally administered levothyroxine from the gastrointestinal tract ranges from 40 to 80%, with the majority of the drug absorbed from the jejunum and upper ileum. Levothyroxine absorption is increased by fasting and decreased in certain malabsorption syndromes, by certain foods, and with age.