[5][8] It is used to treat thyroid hormone deficiency (hypothyroidism), including a severe form known as myxedema coma.

[17] A subset of people with hypothyroidism treated with an appropriate dose of levothyroxine will describe continuing symptoms despite TSH levels in the normal range.

[18] It is also used to treat myxedema coma, which is a severe form of hypothyroidism characterized by mental status changes and hypothermia.

[14] As it is a medical emergency with a high mortality rate, it should be treated in the intensive-care unit[14] with thyroid hormone replacement and aggressive management of individual organ system complications.

[13] Dosages vary according to the age groups and the individual condition of the person, body weight, and compliance with the medication and diet.

Other predictors of the required dosage are sex, body mass index, deiodinase activity (SPINA-GD), and etiology of hypothyroidism.

[23] Poor compliance in taking the medicine is the most common cause of elevated TSH levels in people receiving appropriate doses of levothyroxine.

A nationwide cohort study showed that 1.39% of all pregnant women in 2010 in Denmark received a prescription of levothyroxine during pregnancy.

[24] Given that no increased risk of congenital abnormalities has been demonstrated in pregnant women taking levothyroxine, therapy should be continued during pregnancy.

[18] Since adequate thyroid hormone concentrations are required to maintain normal lactation, appropriate levothyroxine doses should be administered during breastfeeding.

[21][27][28] Overdose can result in heart palpitations, abdominal pain, nausea, anxiousness, confusion, agitation, insomnia, weight loss, and increased appetite.

Acute overdose may cause fever, hypoglycemia, heart failure, coma, and unrecognized adrenal insufficiency.

Substances that reduce absorption are aluminium- and magnesium-containing antacids, simethicone, sucralfate, cholestyramine, colestipol, and polystyrene sulfonate.

[30] Grapefruit juice may delay the absorption of levothyroxine, but based on a study of 10 healthy people aged 20–30 (eight men, two women), it may not have a significant effect on bioavailability in young adults.

[31][32] A study of eight women suggested that coffee may interfere with the intestinal absorption of levothyroxine, though at a level less than eating bran.

Combination of levothyroxine with ketamine may cause hypertension and tachycardia;[34] and tricyclic and tetracyclic antidepressants increase its toxicity.

[35] 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.

[24] 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.

[18] Thyroxine was first isolated in pure form in 1914 at the Mayo Clinic by Edward Calvin Kendall from extracts of hog thyroid glands.

[10][11] Levothyroxine for systemic administration is available as an oral tablet, an intramuscular injection, and as a solution for intravenous infusion.

[14] For people who do switch products, their TSH and free T4 levels should be tested after six weeks to check that they are within the normal range.

[14] Brand names include Eltroxin, Euthyrox, Eutirox, Letrox, Levaxin, Lévothyrox, Levoxyl, L-thyroxine, Thyrax, and Thyrax Duotab in Europe; Thyrox and Thyronorm in South Asia; Euthyrox, Levoxyl, Synthroid, Tirosint, and Unithroid in North and South America; and Thyrin and Thyrolar in Bangladesh.