The standard full replacement dose of levothyroxine is about 1.6 mcg per kilogram of body weight per day, or 0.73 mcg per pound. For a 150 lb adult that is roughly 109 mcg, which rounds to the 112 mcg tablet. Healthy adults under 65 can usually start there. Older adults, people with heart disease, and people with mild (subclinical) hypothyroidism start lower, at 12.5 to 50 mcg, and step up by 12.5 to 25 mcg every 6 to 8 weeks until TSH settles into range.
Levothyroxine is one of the most prescribed medications in the country, and its dosing follows a clearer set of rules than most. The starting point is body weight. From there, age, heart health, how underactive the thyroid is, and pregnancy each move the number. This page walks through the same rules a clinician uses, so the dose on your bottle makes sense.
The 1.6 mcg/kg rule, and where it bends
A thyroid gland that has stopped working needs roughly 1.6 micrograms of levothyroxine per kilogram of body weight each day. That number appears in the FDA prescribing information for Synthroid and in the American Thyroid Association treatment guidelines, and it holds up well for adults with overt hypothyroidism. Multiply your weight in kilograms by 1.6, round to the nearest tablet, and you have the full replacement estimate the calculator shows.
It bends in a few predictable ways. The rule assumes the thyroid contributes nothing, so someone whose gland still makes some hormone, as in early Hashimoto's disease, often needs less. People who have had their thyroid removed or ablated need the whole amount, and sometimes a little more. Body composition matters too: levothyroxine needs track lean mass more closely than total weight, so the rule tends to overshoot in people with a high body fat percentage. Rather than a formula for that, clinicians simply start with the estimate and let TSH correct it.
Why the starting dose is not always the full dose
Thyroid hormone speeds the heart and increases its workload. In a healthy 35-year-old that is harmless and the full dose can start on day one. In someone over 65, or anyone with coronary artery disease, heart failure, or a history of atrial fibrillation, a sudden full dose can provoke chest pain or an irregular rhythm. The FDA label and ATA guidelines both call for a lower start in those groups, typically 12.5 to 25 mcg a day, with increases every 6 to 8 weeks until TSH normalizes.
| Situation | Usual starting dose | Where it heads |
|---|---|---|
| Under 65, healthy heart, overt hypothyroidism | Full weight-based dose (1.6 mcg/kg) | Fine-tuned by 12.5 to 25 mcg after the first TSH |
| 65 or older, or heart disease | 12.5 to 25 mcg | Stepped up every 6 to 8 weeks toward the weight-based dose |
| Subclinical (TSH mildly high, T4 normal) | 25 to 50 mcg | Often settles at 25 to 75 mcg, below full replacement |
| No thyroid (surgery or radioactive iodine) | Full weight-based dose | Sometimes 1.7 mcg/kg or more; the gland contributes nothing |
| Pregnant | Started promptly; needs run 20 to 30% higher | TSH checked every 4 weeks through mid-pregnancy |
Subclinical hypothyroidism is its own case. TSH is above range but free T4 is still normal, which means the gland is keeping up, just under strain. Not everyone with subclinical hypothyroidism needs treatment at all. Those who do usually need far less than full replacement, and 25 to 50 mcg is a typical start.
Adjusting after your TSH comes back
The first TSH on a new dose is drawn 6 to 8 weeks later. Levothyroxine has a half-life of about a week, so it takes five or six weeks for a new dose to reach a steady level and for the pituitary to respond. A TSH drawn at week two is still moving and will mislead you.
From there the adjustments are small. If TSH is still above range, the dose goes up by 12.5 to 25 mcg. If it has dropped below range, the dose comes down by the same step. Each change earns another 6 to 8 week wait and another TSH. Most adults aim for a TSH between about 0.4 and 4.0 mIU/L; the ATA notes that a slightly higher target is reasonable in older adults, and pregnancy uses tighter trimester-specific ranges. Once the number is stable, checks stretch out to every 6 to 12 months. Our guide to TSH and thyroid labs covers how to read the report itself.
One pattern deserves a closer look: a dose well above 1.6 mcg/kg with a TSH that stays high. That usually means the tablet is not being absorbed, or not being taken consistently, rather than the dose being too small. Calcium, iron, antacids, acid-reducing medications, celiac disease, and taking the pill with breakfast are the usual culprits. The calculator flags this combination so it can be sorted out before the dose climbs further.
Do not change your levothyroxine dose on your own. Contact your clinician promptly if any of these apply:
- Chest pain, a racing or irregular heartbeat, or shortness of breath after a dose change
- A TSH below 0.1 mIU/L, which raises the risk of atrial fibrillation and bone loss over time
- You are pregnant or just found out you are; dosing and TSH targets change immediately
- Severe fatigue, confusion, a very slow heart rate, or feeling cold and puffy, which can signal dangerously low thyroid levels
- You have adrenal insufficiency or symptoms of it; levothyroxine can trigger a crisis if the adrenals are not treated first
This calculator is a way to understand the numbers, not a substitute for the clinician who prescribes and monitors them.
Taking it so the dose actually works
Levothyroxine is absorbed poorly when it competes with food or minerals, and a dose that looks right on paper can behave like a smaller one. The label directs a single daily dose on an empty stomach, 30 to 60 minutes before breakfast, with calcium, iron, antacids, and similar supplements kept at least 4 hours away. Coffee counts as food for this purpose. Bedtime dosing, at least 3 hours after the last meal, works equally well for people who cannot manage the morning gap. Consistency matters more than the exact time, because a steady routine makes your TSH readings mean something.
Two more details trip people up. Biotin, common in hair and nail supplements, interferes with the lab assay and can make TSH look falsely low; stop it for two days before a blood draw. And switching between brands or generics can shift absorption slightly, so the ATA recommends staying on one product where possible and rechecking TSH after any switch. The levothyroxine treatment guide covers timing, interactions, and what to expect in the first weeks.
How this calculator works
In starting mode, it converts your weight to kilograms and multiplies by 1.6 mcg/kg to estimate full replacement, then rounds to the nearest of the 12 tablet strengths (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg). If you are 65 or older or have marked heart disease, it suggests a 12.5 to 25 mcg start instead and shows the full dose as the destination. Subclinical hypothyroidism gets a 25 to 50 mcg start, or 12.5 to 25 mcg in the cautious group. In adjusting mode, it compares your TSH with a target of 0.4 to 4.0 mIU/L (an upper limit of 2.5 during pregnancy), suggests a 12.5 or 25 mcg step in the right direction, picks the nearest tablet strength, and shows your current dose per kilogram beside the weight-based reference. The dosing rules come from the FDA prescribing information and the American Thyroid Association guidelines cited below. It is built for adults; children are dosed on a different scale. Everything runs in your browser, and nothing you enter is transmitted or stored.
Common questions
For an adult with overt hypothyroidism, the full replacement dose is about 1.6 micrograms per kilogram of body weight per day (0.73 mcg per pound). That figure comes from the FDA prescribing information and the American Thyroid Association guidelines. A 150 lb (68 kg) adult works out to about 109 mcg, which rounds to the 112 mcg tablet. The calculator uses that rule, then adjusts the starting dose for age, heart disease, and how underactive the thyroid is.
Thyroid hormone raises heart rate and the heart's workload. In adults over about 65, or anyone with coronary disease, heart failure, or a risk of atrial fibrillation, jumping straight to a full dose can trigger angina or an irregular rhythm. The FDA label and ATA guidelines call for a lower start, usually 12.5 to 25 mcg a day, then increases every 6 to 8 weeks. Healthy younger adults can usually start at the full weight-based dose.
Six to eight weeks. Levothyroxine has a long half-life, about a week, so it takes roughly five to six weeks for a new dose to reach steady state and for TSH to settle. Testing sooner shows a number that is still moving. Once you are stable, TSH is usually checked every 6 to 12 months, or sooner if symptoms change, you become pregnant, or your weight shifts a lot.
Usually yes, and early. Levothyroxine needs rise 20 to 30 percent in pregnancy, often before the first prenatal visit. The ATA advises people on levothyroxine to take two extra doses a week as soon as pregnancy is confirmed and to check TSH every 4 weeks through mid-pregnancy. The calculator flags this, but pregnancy dosing should be managed directly with your clinician or obstetrician.
The microgram strengths are the same, and the same weight-based rules apply. Small differences in absorption between products can shift TSH slightly, so the ATA recommends staying on one product and rechecking TSH if the pharmacy switches you. The tablet colors are standardized by strength across most brands and generics, which makes it easy to confirm you were given the right one.
- FDA (DailyMed, National Library of Medicine). SYNTHROID (levothyroxine sodium) tablets: full prescribing information, Dosage and Administration (2024)
- American Thyroid Association (Jonklaas J, et al.), Thyroid. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement (2014)
- American Thyroid Association (Alexander EK, et al.), Thyroid. 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum (2017)
- MedlinePlus (NIH). Levothyroxine: Drug Information
- NIDDK (NIH). Hypothyroidism (Underactive Thyroid)