TSH Dose Calculator: Is Your Levothyroxine Dose Right For Your Target?
Pick the TSH you're aiming for and find out whether your current dose can get you there:
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: August 28, 2026
How To Use This Calculator:
#1. Enter Your Dose And Your TSH
Your current levothyroxine dose and your most recent TSH. Add your weight and you get the per-kilogram check doctors use too.
#2. Pick Your Target
The same targets as my dose adjustment calculator: my recommended optimal range of 0.5 to 1.0, the functional range of 0.5 to 2.5, or the pregnancy target. The same TSH reads completely differently depending on where you’re aiming.
#3. Read The Verdict
Room to increase, possibly too much, on target, or too soon to judge, with the step size attached: 12.5, 25, or 50 mcg depending on how far your TSH sits from target, capped at 12.5 if you’re 65 or older or have a heart condition.
Why The Target Matters More Than The Number
A TSH of 3.5 is “normal” on every lab report in America. It’s also a TSH at which a large share of thyroid patients feel terrible.
That’s the whole story of this calculator: the reference range (roughly 0.4 to 4.5) describes the population, and the optimal band describes where treated patients tend to actually feel well, which in my experience is a TSH between 0.5 and 1.0.[1]
The mechanics are simple. Doctors adjust levothyroxine in 12.5 to 25 mcg steps, then wait, because TSH takes 6 to 8 weeks to settle after any change.[2] A TSH drawn 3 weeks after a dose change reflects the old dose and misleads everyone.
And the weight check matters because full replacement runs about 1.6 mcg per kilogram of body weight.[2] A dose far above that estimate with a stubborn TSH usually means an absorption problem (coffee, calcium, iron, timing), and fixing the absorption beats raising the dose.
Frequently Asked Questions
The lab’s normal range is roughly 0.4 to 4.5, but treated patients most often report feeling best with a TSH between 0.5 and 1.0. If your TSH is 3.8 and your doctor says you’re fine but you feel awful, the range disagreement is the conversation to have, and free T3 is the next lab to look at.
It depends on how far your TSH sits from your target. Within about twice the target: 12.5 mcg. Up to five times: 25 mcg. Beyond that: 50 mcg. And a heart condition or age 65 and older caps every step at 12.5, because overshooting swings you hyper and every change costs 6 to 8 weeks of waiting to see the true result.
Levothyroxine has a half-life of about a week, so it takes over a month for blood levels to fully reflect a new dose, and your pituitary needs time on top of that to adjust its TSH output. Testing early gives you a number that reflects the old dose, and decisions made on it are guesses.
Then TSH has told you everything it can, and the question moves downstream: is your body converting T4 into enough active T3? Ask for free T3 and reverse T3 on your next draw, and put the results through the conversion calculator. This exact situation is the most common unsolved case in thyroid treatment.
Related Tools
- Levothyroxine Dose Adjustment Calculator: the adjustment rules on their own
- Optimal Thyroid Lab Calculator: every lab, not just TSH
- T4 to T3 Conversion Calculator: when TSH looks fine but you feel bad
- All Thyroid Calculators & Quizzes: the full set of free tools
Scientific References
- Wartofsky L, Dickey RA. The evidence for a narrower thyrotropin reference range is compelling. J Clin Endocrinol Metab. 2005;90(9):5483-5488.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751.
- Alexander EK, Pearce EN, Brent GA, 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(3):315-389.