How levothyroxine dosing works
Levothyroxine is synthetic thyroxine (T4), the standard treatment for hypothyroidism, and it has a narrow therapeutic window: too little leaves the patient hypothyroid, too much causes subclinical or overt thyrotoxicosis with risks to the heart and bones. Because the body's daily T4 production scales with body size, replacement dosing is weight-based. For an adult with overt primary hypothyroidism and little or no functioning thyroid tissue left, the FDA-approved labelling and the American Thyroid Association (ATA) guidelines agree on the full replacement dose: approximately 1.6 mcg per kilogram of body weight per day. The arithmetic is simple, which is why people search for a "TSH dose calculator": weight in kilograms times 1.6 gives the estimated daily dose in micrograms. A 70 kg adult needs about 112 mcg per day; a 60 kg adult about 96 mcg; a 90 kg adult about 144 mcg.
TSH itself does not set the starting dose. TSH is the feedback signal the pituitary uses to report whether the body has enough thyroid hormone, and it is the test used to steer treatment after the start: checked about 6 weeks after beginning or changing the dose, then used to fine-tune in small steps. Understanding that division of labour, weight for the estimate and TSH for the adjustment, prevents the commonest misunderstanding of levothyroxine therapy, which is expecting a formula to convert a TSH value directly into a dose. No validated formula does that, because the same TSH can reflect very different residual thyroid function in different people.
Rounding to real tablet strengths
Levothyroxine is manufactured in fixed tablet strengths, and the calculated dose almost never lands exactly on one. The strengths in routine use are 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200 and 300 mcg; the unusual values in that list (88, 112, 137) exist precisely because the standard weight-based doses kept falling between round numbers. This calculator rounds the estimate to the nearest available strength and states the rounding openly: 96 mcg becomes the 100 mcg tablet, a difference of 4 mcg (about 4 percent), while 80 mcg becomes 75 mcg because that is closer than 88. A rounding difference is not an error to hide; the 6-week TSH check exists to catch its effect, and clinicians fine-tune further with 12.5 to 25 mcg increments, occasionally by using different strengths on different days of the week to achieve an average dose between two strengths. Note that doses above 200 mcg per day are seldom required; an apparent need for much more should prompt a review of adherence, absorption and interacting medicines rather than indefinite escalation, as the FDA label itself points out.
| Weight | Exact estimate (x 1.6 mcg/kg) | Nearest tablet strength | Rounding |
|---|---|---|---|
| 50 kg | 80 mcg | 75 mcg | 5 mcg below the estimate |
| 60 kg | 96 mcg | 100 mcg | 4 mcg above |
| 70 kg | 112 mcg | 112 mcg | Exact match |
| 80 kg | 128 mcg | 125 mcg | 3 mcg below |
| 90 kg | 144 mcg | 150 mcg | 6 mcg above |
| 100 kg | 160 mcg | 150 mcg | 10 mcg below (175 mcg is 15 above) |
Starting low: older patients and cardiac disease
The full replacement dose is a starting point for younger, otherwise healthy adults. It is not a safe first dose for everyone. Thyroid hormone increases heart rate, contractility and myocardial oxygen demand, and in a patient with coronary artery disease, heart failure or a risk of atrial fibrillation, jumping straight to full replacement can precipitate angina, arrhythmia or decompensation. The FDA label directs a lower starting dose with slower titration for these patients and for geriatric patients generally, and ATA guidance puts the figure at 12.5 to 25 mcg daily to start. From there the dose is increased gradually, typically by 12.5 to 25 mcg every 4 to 6 weeks or more slowly, checking TSH and symptoms at each step. The calculator's low-start group shows both numbers honestly: the cautious starting range you would actually use first, and the weight-based estimate labelled as the expected eventual requirement, not the first prescription. Reaching the full dose may take several months, and some older patients settle appropriately on less than the formula suggests, because dose requirement falls with age.
Pregnancy: the dose goes up, early
Pregnancy increases levothyroxine requirements by about 20 to 30 percent, driven by rising thyroxine-binding globulin, placental deiodinase activity and transfer of T4 to the fetus, and the increase happens early, often by the fifth week of gestation. Because maternal hypothyroidism in early pregnancy carries fetal risks, ATA guidance supports increasing the dose as soon as pregnancy is confirmed rather than waiting for TSH to drift up. A practical method the ATA describes is to take two additional doses per week, nine tablets instead of seven, which raises the weekly total by about 29 percent without changing the tablet strength: a woman on 100 mcg daily takes 900 mcg per week, an average of about 129 mcg per day. Alternatively the daily dose can be raised by 20 to 30 percent and rounded to a strength, which is what the calculator shows for the weight-based estimate. TSH is then monitored roughly every 4 weeks during the first half of pregnancy against trimester-specific targets, and the dose is usually reduced back to the pre-pregnancy dose immediately after delivery, with a TSH check around 6 weeks postpartum.
Monitoring: TSH at 6 weeks, small steps
Levothyroxine has a half-life of about a week, so blood levels and the pituitary's TSH response take roughly 6 weeks to stabilise after any change. Checking TSH at 2 weeks and reacting to it is a recipe for overshooting in both directions. The standard rhythm, reflected in the FDA label, is: start, wait about 6 weeks, check TSH (with free T4 where indicated), adjust by 12.5 to 25 mcg, wait another 4 to 6 weeks, and repeat until TSH sits in the target range. Once stable, monitoring stretches to every 6 to 12 months, and restarts whenever something changes: a dose or brand change, major weight change, pregnancy, new interacting medication, or symptoms suggesting under- or over-replacement. In central hypothyroidism, where the problem is pituitary rather than thyroid, TSH cannot be used this way at all and dosing is guided by free T4, a distinction any TSH dose calculator has to state plainly.
Absorption: the dose only counts if it gets in
A correctly calculated dose can still underperform if absorption is poor. Levothyroxine is absorbed best on an empty stomach and is conventionally taken 30 to 60 minutes before breakfast, or at bedtime several hours after the last meal. Iron and calcium supplements, and several other medicines and foods including soy products and coffee taken at the same time, reduce absorption and should be separated from the tablet by several hours where the product information advises it. Conditions affecting the gut, and acid-suppressing medication, can also change the requirement over time. When a patient's TSH will not settle despite an apparently adequate dose, the review sequence is adherence first, then timing and interactions, then malabsorption, before assuming the weight-based estimate itself is wrong.
When the full replacement estimate does not apply
Three situations deserve explicit honesty. First, subclinical hypothyroidism (raised TSH, normal free T4) and partial thyroid failure need much less than 1.6 mcg/kg, commonly 25 to 75 mcg daily, because the gland still produces part of the requirement. Second, after thyroidectomy for thyroid cancer, the aim may be deliberate TSH suppression, which typically needs more than the replacement dose; the label notes this usually requires more than 2 mcg/kg per day, under specialist supervision. Third, children and adolescents dose differently and are outside this calculator's scope; paediatric dosing belongs with the paediatric references and tools. In every case the number on this page is an estimate to discuss with the prescribing clinician, who knows the TSH, free T4, diagnosis and cardiac history that a weight-only calculation cannot see. For other weight-based medicines, the general drug dose calculator applies the same arithmetic with a maximum dose cap.
References
Levothyroxine sodium tablets, FDA-approved prescribing information (DailyMed): full replacement dose 1.6 mcg/kg/day in adults; titration in 12.5 to 25 mcg increments every 4 to 6 weeks; lower starting dose and slower titration for geriatric patients and those with cardiac disease or atrial fibrillation risk; dosages greater than 200 mcg/day are seldom required.
Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the Treatment of Hypothyroidism. Thyroid. 2014;24(12):1670-1751. American Thyroid Association guidance on replacement dosing, low starting doses in elderly and cardiac patients, and monitoring.
Alexander EK, Pearce EN, Brent GA, Brown RS. 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. Source of the 20 to 30 percent pregnancy increase and the two-additional-doses-per-week method.