Levothyroxine is a synthetic form of thyroxine (T4), the main hormone made by the thyroid gland. It is sold as Synthroid, Levoxyl and generic tablets.
Also known as: Synthroid, Levoxyl, T4.
Synthroid tablets come in 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 and 300 mcg strengths.
The FDA Synthroid label approves levothyroxine for hypothyroidism (primary, secondary or tertiary, present at birth or acquired) in adults and children, and to suppress TSH alongside surgery and radioactive iodine in certain thyroid cancers. It is not approved for benign thyroid nodules or goiter in people with enough iodine.
The label says to take it once daily on an empty stomach, half an hour to an hour before breakfast, and at least 4 hours apart from drugs that interfere with absorption, such as iron, calcium, antacids, bile acid sequestrants and phosphate binders. The full adult replacement dose is about 1.6 mcg per kg per day, adjusted in steps of 12.5 to 25 mcg every 4 to 6 weeks, with lower starting doses for older people and people with heart disease. Full effect can take 4 to 6 weeks.
The label carries a boxed warning: not for treatment of obesity or for weight loss. Soybean flour, walnuts and dietary fiber can reduce absorption.
The FDA Synthroid label gives a half-life for levothyroxine of 6 to 7 days in people with normal thyroid function, 3 to 4 days in hyperthyroidism and 9 to 10 days in hypothyroidism; Dose Track uses 7 days. The label gives no time to peak. In a study of healthy men given 100 mcg (Leggio 2006), levels peaked about 2.7 hours after a dose; the app uses 2.5 hours.
Numbers shown are the values Dose Track uses, listed only where they agree with the source above. Half-life varies from person to person and with injection site.