What is levothyroxine?
Levothyroxine (T4) is a synthetic form of thyroxine, the main hormone the thyroid gland normally produces. It's the standard first-line treatment for hypothyroidism (an underactive thyroid) — taken once daily, it replaces the hormone the gland isn't making enough of, restoring normal metabolic function. It's also used after thyroidectomy or radioactive iodine treatment, when the thyroid gland has been removed or destroyed and can't produce hormone at all.
How the dose is calculated
This is rounded to the nearest standard commercial tablet strength (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, 300 mcg) since levothyroxine isn't typically dosed in arbitrary increments.
Normal starting doses
| Patient profile | Typical starting dose |
|---|---|
| Healthy adult, < ~60 years, no cardiac disease | Full calculated replacement dose (~1.6 mcg/kg/day) |
| ≥ 60 years, or known coronary artery disease | 25 mcg/day, titrated slowly upward |
| Mild / subclinical hypothyroidism | 25–75 mcg/day depending on TSH elevation |
| Post-thyroidectomy (full replacement needed) | Often toward the higher end, ~1.6–1.8 mcg/kg/day |
Doses are adjusted in 12.5–25 mcg increments, guided by a repeat TSH roughly 6–8 weeks after any dose change — not before, since TSH takes that long to fully re-equilibrate.
Clinical use
- "Start low, go slow" in older adults and cardiac disease: a full replacement dose started abruptly can precipitate angina or arrhythmia in patients with underlying coronary disease, so this group starts at a low dose regardless of their calculated full-replacement requirement.
- Take on an empty stomach: levothyroxine absorption is significantly reduced by food, coffee, calcium, iron, and some antacids — standard advice is 30–60 minutes before breakfast, consistently.
- Pregnancy: levothyroxine requirements typically rise by roughly 20–30% (sometimes more) as soon as pregnancy is confirmed — this calculator is for non-pregnant adult dosing; pregnant patients need closer specialist monitoring and their own dose-adjustment protocol.
- Not for paediatric or neonatal dosing: children (especially congenital hypothyroidism) are dosed very differently (much higher mcg/kg, decreasing with age) — this tool covers adult dosing only.
Frequently asked questions
What is levothyroxine?
It's a synthetic (man-made) version of thyroxine (T4), the hormone your thyroid gland normally produces. It's used to treat hypothyroidism — whether from thyroid disease, surgical removal, or radioactive iodine treatment — by replacing the hormone the body isn't making enough of on its own.
Is 25 mcg of levothyroxine a high dose?
No — 25 mcg is the lowest standard starting dose, typically used to begin therapy cautiously in older adults, people with cardiac disease, or mild/subclinical hypothyroidism. It's well below full replacement for almost every adult (full replacement is usually 100–150+ mcg/day for a typical adult body weight), so 25 mcg is considered a low, conservative dose, not a high one.
Is 50 mcg of levothyroxine a high dose?
No — 50 mcg is a low-to-moderate dose. It's a common starting dose or an early step up from 25 mcg, but it's still below the full calculated replacement dose for most average-sized adults (roughly 1.6 mcg/kg/day, which works out above 50 mcg for anyone over about 30 kg). Many patients are titrated well past 50 mcg to reach their target TSH.
Is 100 mcg of levothyroxine a high dose?
Generally no. 100 mcg sits close to full replacement for a smaller adult (around 60–65 kg at 1.6 mcg/kg/day) but is still a moderate, sub-full-replacement dose for an average or larger adult, where full replacement is often 125–175 mcg/day or more. Whether 100 mcg is "enough," "high," or "low" for a specific person really depends on their body weight, residual thyroid function, and — most importantly — their TSH result, not the number alone. Doses above roughly 200 mcg/day are more where "high dose" starts to apply for standard replacement.
How is the levothyroxine starting dose decided?
Mainly by body weight, age, and cardiac risk. Healthy adults under about 60 with no heart disease can usually start at the full weight-based calculated dose. Older adults or those with known or suspected cardiac disease start much lower (25 mcg/day) and titrate up gradually, because a sudden jump to full replacement can stress the heart.
How often should TSH be rechecked after starting or changing the dose?
Roughly 6–8 weeks after any dose change — TSH takes that long to fully reflect the new dose, so checking earlier can be misleading and lead to over-correction.
Why do older adults and people with heart disease start at a lower dose?
Thyroid hormone increases the heart's oxygen demand and metabolic rate. Starting at full replacement in someone with reduced cardiac reserve or coronary artery disease can precipitate angina, arrhythmia, or worse — so these patients start low and are titrated up slowly and carefully instead.
Does the levothyroxine dose need to change during pregnancy?
Usually yes — thyroid hormone requirements typically increase by roughly 20–30% (sometimes more) very early in pregnancy, and under-treatment during pregnancy has real risks for both mother and baby. Pregnant patients on levothyroxine need closer, specialist-guided monitoring rather than the standard non-pregnant titration schedule.
References
- Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24:1670–1751.
- Garber JR, et al. Clinical Practice Guidelines for Hypothyroidism in Adults (AACE/ATA). Endocr Pract. 2012;18:988–1028.
- Alexander EK, et al. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid. 2017;27:315–389.