Levothyroxine
Levothyroxine is the synthetic form of the thyroid hormone thyroxine (T4), used as standard replacement therapy in hypothyroidism. It has a narrow therapeutic index and absorption is sensitive to food, calcium and iron, requiring careful dosing and timing.
- Chemical formula
- C15H11I4NO4
- CAS number
- 51-48-9
- ATC code
- H03AA01
- Molecular weight
- 776.87 g/mol
- Drug class
- Thyroid hormone replacement
- Also known as
- Synthroid, Levoxyl, Euthyrox, Eltroxin, T4
What is it?
Levothyroxine is synthetic thyroxine (T4), the principal hormone of the thyroid gland, in clinical use since the 1950s. It is on the WHO Essential Medicines List and is among the most prescribed medicines worldwide. Marketed under many brand names including Synthroid (Abbott/AbbVie), Levoxyl, Euthyrox and Eltroxin, with authorised generics widely available. It has a long, well-characterised efficacy and safety record across decades of clinical use.
Mechanism of action
Levothyroxine replaces deficient endogenous thyroxine, which is converted in tissues to the active hormone triiodothyronine (T3) by deiodinase enzymes. Adequate replacement restores normal thyroid hormone signalling at nuclear receptors, normalising metabolic rate, cardiovascular function, growth and development. The therapeutic goal is restoration of euthyroidism judged by TSH (and free T4) within laboratory reference ranges.
Pharmacokinetics
Oral absorption is 60–80% on an empty stomach but is reduced by food, coffee, calcium, iron and proton pump inhibitors. The terminal half-life is ~7 days, supporting once-daily dosing and producing stable plasma levels. Hepatic and tissue deiodination yields T3. Steady state is reached after 4–6 weeks. The narrow therapeutic index means small dose changes can swing TSH meaningfully, and brand-to-generic switches are managed cautiously.
Indications
Levothyroxine is approved for hypothyroidism of any cause (Hashimoto thyroiditis, post-thyroidectomy, post-radioiodine, congenital), goitre and TSH suppression after differentiated thyroid cancer. Subclinical hypothyroidism may be treated when TSH is persistently elevated, especially in pregnancy. According to current guidelines, levothyroxine alone is the standard first-line therapy; combination T4/T3 or desiccated thyroid extract are not recommended as routine first-line.
Safety profile
At correct dose, levothyroxine has minimal adverse effects because it replaces a hormone the body normally produces. Over-replacement causes thyrotoxic symptoms — palpitations, tremor, weight loss, heat intolerance, anxiety, insomnia — and long-term over-dosing accelerates bone loss and risk of atrial fibrillation, particularly in older adults. Under-replacement leaves persistent hypothyroid symptoms. TSH monitoring 6–8 weeks after any dose change guides titration.
Products containing this ingredient
Frequently asked questions
Why must levothyroxine be taken on an empty stomach? ▾
Food, coffee, calcium and iron substantially reduce levothyroxine absorption, leading to subtherapeutic levels and persistent hypothyroidism. According to the prescribing information, levothyroxine should be taken at least 30–60 minutes before breakfast or 4 hours after the last meal, with water only. Consistent timing is more important than perfect timing — pick a routine and keep it.
Can I switch between levothyroxine brands or to a generic? ▾
Yes, but careful TSH re-checking after any switch is recommended because the narrow therapeutic index means small bioavailability differences between products can shift TSH out of range. According to clinical guidance, once a stable dose is reached on a particular product, that product should be continued where possible, and TSH re-tested 6–8 weeks after any switch.
Why is dose-adjusted in pregnancy? ▾
Thyroid hormone requirement increases by ~25–30% in pregnancy because of higher thyroxine-binding globulin and fetal demand. Women on stable replacement should increase dose at confirmation of pregnancy and have TSH checked every 4 weeks during the first trimester. According to obstetric guidelines, target TSH in pregnancy is below 2.5 mIU/L in the first trimester.
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.