Last updated: March 2026
The most prescribed medication in the United States. Synthetic thyroxine — your body converts it to active T3. First-line treatment for hypothyroidism since the 1950s.
Levothyroxine (synthetic T4/thyroxine) is the standard treatment for hypothyroidism. It provides the prohormone that your body naturally converts to the active T3 hormone.
Levothyroxine is a prohormone. Your body converts it to active T3 via deiodinase enzymes in tissues. This conversion is why T4 alone works for most patients.
Thyroid hormone (via T4 supplementation) suppresses TSH from the pituitary. TSH is the main lab marker used to titrate levothyroxine dose.
T4 has a half-life of ~7 days, allowing once-daily dosing. Full equilibrium takes 6-8 weeks after dose changes. Stable levels are easier to maintain than T3.
Levothyroxine absorption is affected by calcium, iron, fiber, coffee, and many medications. Take on empty stomach, 30-60 min before food/supplements.
Context: Levothyroxine is the best-studied thyroid medication. Decades of data show it's effective for most hypothyroid patients. TSH normalization correlates with symptom improvement in most cases.
Support for thyroid medication.
Dosing schedules, interaction warnings, and cycle protocols for 50+ compounds — all in one place.
Levothyroxine is a prescription medication. Must be monitored with regular TSH blood tests. Never adjust dose without medical supervision. Take on empty stomach, separate from supplements. This is not medical advice. FDA Approved Rx Required