Levothyroxine Dosage Calculator
This calculator gives you a weight-based starting dose of levothyroxine (levothyroxine sodium, a synthetic form of the T4 thyroid hormone sold under brand names Synthroid, Tirosint, Tirosint-SOL, Levoxyl, and Unithroid) for hypothyroidism. It’s built for adults with primary hypothyroidism, Hashimoto’s thyroiditis, subclinical hypothyroidism, or patients who’ve had a total or partial thyroidectomy, and it adjusts automatically for the factors that matter most: pregnancy, age, heart conditions, and weight.
Here’s how to use it:
Enter your weight, height, age, sex, and BMI (or ideal BMI if needed), flag any modifiers that apply to you, pick your target medication, and hit calculate. It’s that easy.
The calculator uses the 1.6 mcg/kg/day full-replacement formula from the American Thyroid Association and American Association of Clinical Endocrinologists guidelines[1][2], adjusted for clinical scenario and modifiers.
Note: This calculator is a starting estimate, not a final prescription. Your actual dose will be refined by your TSH and free T4 labs 6 to 8 weeks after starting or changing a dose. Never change your dose without talking to your prescriber first.
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: April 22, 2026.
How to Use This Calculator
Step #1: Pick Your Clinical Scenario
At the top of the calculator, you’ll see five scenarios: new thyroid diagnosis, post-thyroidectomy for benign reasons, post-thyroidectomy for thyroid cancer (where TSH suppression is the goal), subclinical hypothyroidism, and pregnancy.
Each scenario changes how the calculator does the math. Cancer patients after thyroid surgery need a higher dose. Subclinical hypothyroidism needs a lower one. Pregnancy pushes the dose up by 20 to 30%. You don’t need to know the numbers. The calculator handles them.
If you don’t know which one fits, pick “New thyroid diagnosis.” It’s the most common use case and the formula everything else is built on.
Step #2: Enter Your Weight, Height, Age, and Sex
Weight is the only required input. Everything else just gives you a more accurate calculation.
Enter your weight in pounds or kilograms (there’s a toggle). If you’re inside a normal BMI range, that’s all the calculator needs. If you’re overweight or obese, or if you’re not sure, add your height. The calculator will calculate your BMI and automatically switch to ideal body weight (IBW) for the calculation if your BMI is 30 or above. Dosing an overweight patient on actual body weight overshoots the dose, because adipose tissue doesn’t require thyroid hormone at the same rate as muscle mass.
Age matters because the calculator uses a lower, safer starting dose if you’re 65 or older. Older patients clear levothyroxine more slowly and are more sensitive to heart-related side effects, so the starting point is 25 to 50 mcg with smaller dose adjustments over time.
Sex is only used to calculate ideal body weight (the Devine formula gives slightly different numbers for men and women), so if you’ve entered a height, pick your sex.
Step #3: Flag Any Modifiers That Apply
Check the heart condition box if you’ve had a heart attack or heart disease, heart failure, angina (chest pain), or a history of arrhythmia (abnormal heart rhythm). When that’s checked, the calculator overrides the weight-based calculation and drops your starting dose to 25 mcg with a “start low, go slow” note, because rapid thyroid hormone replacement has the potential to trigger chest pain or arrhythmia in patients with an underlying heart condition.
The obesity toggle is usually handled automatically (BMI 30 or above triggers it), but you can flip it on manually if you want the IBW-based calculation regardless of what your BMI shows.
A heart condition takes priority over everything else. Being 65 or older takes priority over scenario-based dosing. A higher BMI changes which weight the calculator uses (actual vs. ideal), but doesn’t change the rest of the calculation.
Step #4: Pick Your Target Medication
Pick the exact medication you’ll be taking from the dropdown. The options are generic levothyroxine, Synthroid, Tirosint (gelcap), Tirosint-SOL (liquid), Levoxyl, or Unithroid. Each one comes in a different set of tablet or capsule strengths, and the calculator will round your dose to the nearest available strength for whichever one you pick.
Tirosint and Tirosint-SOL come in more dosing strengths than standard tablets, which makes them useful if your calculated dose lands between two standard tablet strengths. Tirosint-SOL in particular can be dosed in drops, giving you the most precise dose adjustment of any levothyroxine product.
If you’re weighing the brand differences (Synthroid vs generic, Tirosint vs tablets), or considering whether natural desiccated thyroid might be a better fit for you, read my Armour Thyroid vs Synthroid comparison for a side-by-side breakdown.
Hit calculate. Change any input and the calculator updates immediately.
Understanding Your Results
Your BMI and Effective Weight
If you entered your height, the first box shows your BMI and the weight the calculator is actually using for the dose calculation.
If your BMI is under 30, the calculator uses your actual body weight. If your BMI is 30 or above, it switches to your ideal body weight. This is the standard approach in guideline-based dosing[1], because fat tissue doesn’t use thyroid hormone at the same rate as muscle.
If you didn’t enter your height, the calculator assumes normal BMI and uses your actual weight. If you know you’re overweight or obese, go back and add your height for a more accurate starting estimate.
Your Calculated Starting Dose
This is the dose your weight and clinical scenario point to, shown in micrograms (mcg) per day. Levothyroxine is always dosed in micrograms, which is 1/1000 of a milligram, so a 100 mcg dose is 0.1 mg.
The number you see is your weight multiplied by a factor based on which scenario you picked. A new diagnosis in a healthy adult uses the standard dose. Cancer after thyroid surgery needs a higher dose to keep TSH suppressed. Subclinical hypothyroidism needs a lower dose. Pregnancy needs a higher dose. The exact multipliers live in the FAQs below.
If you check the heart condition box, the dose is 25 mcg no matter what you weigh. If you’re 65 or older without a heart condition, it caps at 50 mcg or your weight-based dose rounded down, whichever is lower.
Nearest Commercially Available Strength
Levothyroxine comes in specific tablet strengths. Standard tablets (generic Levothyroxine, Synthroid, Levoxyl, Unithroid) come in 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, and 200 mcg. Tirosint capsules come in 13, 25, 50, 75, 88, 100, 112, 125, 137, and 150 mcg. Tirosint-SOL adds 37.5, 62.5, and the rest up to 200 mcg.
The calculator rounds your calculated dose to the closest strength available for the medication you picked. That’s the simplest starting prescription.
Closer Match With 2 Tablets
If combining two tablet strengths lands closer to your ideal dose than any single tablet can get, the calculator will let you know.
For example: a 75 kg patient with a new hypothyroidism diagnosis needs about 120 mcg. The closest single standard tablet is 125. But a 100 plus a 25 gives you exactly 125 as well, while a 112 plus nothing gets you to 112. The 2-tablet combo is sometimes the only way to land on an unusual target without a compound prescription.
This box only appears when the 2-tablet combination is meaningfully closer. If the nearest single tablet is already within a few micrograms of your ideal, you won’t see it.
Scenario-Specific Dosing Notes
The last section gives you clinical notes that change based on the scenario and modifiers you picked.
For pregnancy, you’ll see the first-trimester TSH target of below 2.5 mU/L, the recommendation to recheck TSH every 4 weeks through the first half of pregnancy, and a reminder to return to your pre-pregnancy dose right after delivery. For patients with a heart condition, you’ll see a “start low, go slow” note and the instruction to report chest pain or a racing heartbeat to your prescriber immediately. For post-thyroidectomy cancer patients, you’ll see the TSH target based on your risk level and the note that endocrinology follow-up is required. For subclinical hypothyroidism, you’ll see the factors that influence whether to treat at all.
Common Levothyroxine Starting Doses by Weight
Here’s a quick-reference table of typical starting doses for the most common clinical scenarios, using the weight-based formulas built into the calculator above. Doses are rounded to the nearest commercially available tablet strength (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, or 200 mcg).
| Body Weight | New Thyroid Diagnosis (1.6 mcg/kg) | Post-Thyroidectomy Benign (1.7 mcg/kg) | Subclinical Hypothyroidism (1.1 mcg/kg) | Pregnancy New Diagnosis (2.2 mcg/kg) |
|---|---|---|---|---|
| 100 lb (45 kg) | 75 mcg | 75 mcg | 50 mcg | 100 mcg |
| 125 lb (57 kg) | 88 mcg | 100 mcg | 75 mcg | 125 mcg |
| 150 lb (68 kg) | 112 mcg | 112 mcg | 75 mcg | 150 mcg |
| 175 lb (79 kg) | 125 mcg | 137 mcg | 88 mcg | 175 mcg |
| 200 lb (91 kg) | 150 mcg | 150 mcg | 100 mcg | 200 mcg |
| 225 lb (102 kg) | 175 mcg | 175 mcg | 112 mcg | 200 mcg |
These numbers assume a normal BMI. If your BMI is 30 or above, the calculator above will switch to ideal body weight automatically, and your starting dose will usually come in lower than what this table shows. That’s the correct adjustment, not a mistake. Using actual body weight at a higher BMI leads to overdosing and a suppressed TSH.
These are also starting estimates. A heart condition, being over 65, and a history of thyroid cancer all change the math (lower starting doses for heart patients and older patients, higher for thyroid cancer). The calculator above applies all of those adjustments for you.
If you’re on Armour Thyroid, NP Thyroid, or another non-levothyroxine medication and you want the equivalent dose, use the Thyroid Medication Conversion Calculator instead.
Frequently Asked Questions
The standard full-replacement dose of levothyroxine is 1.6 mcg per kilogram of body weight per day, or about 0.73 mcg per pound, per the American Thyroid Association and American Association of Clinical Endocrinologists guidelines[1]. For a 150 pound (68 kg) adult, that works out to about 112 mcg per day. For a 200 pound (91 kg) adult, it’s about 150 mcg per day.
The formula changes depending on the scenario. Post-thyroidectomy cancer patients need more (closer to 2.0 mcg/kg for TSH suppression), subclinical hypothyroidism patients need less (about 1.0 to 1.2 mcg/kg), and pregnancy increases the requirement by 20 to 30%. Enter your weight and clinical scenario in the calculator above for an exact number.
For a healthy adult under 60 with new primary hypothyroidism, the standard starting dose of levothyroxine is the full weight-based dose of 1.6 mcg/kg/day, which typically lands between 100 and 150 mcg per day[1]. Most adults don’t need to start at a low dose and titrate up.
Adults over 65, anyone with a known heart condition, and patients with long-standing severe hypothyroidism are the exceptions. In those cases the recommended starting dose is 12.5 to 50 mcg per day, and the dose is adjusted in small steps of 12.5 to 25 mcg every 4 to 6 weeks, because bringing levothyroxine up too fast can trigger chest pain or a racing heartbeat[1]. The calculator above applies that adjustment automatically when you flag a heart condition or enter an age of 65 or older.
After a total thyroidectomy for a benign reason, most patients need 1.6 to 1.7 mcg/kg/day of levothyroxine, which is the full replacement dose for a person without any remaining thyroid function[1]. For a 70 kg adult that’s about 112 to 125 mcg per day.
Thyroid cancer patients needing TSH suppression require more, usually 1.8 to 2.2 mcg/kg/day depending on risk stratification[4]. Low-risk cancer targets a TSH of less than 2.0 mU/L, intermediate-risk targets 0.1 to 0.5, and high-risk targets less than 0.1. Post-thyroidectomy dosing should always be managed with endocrinology follow-up.
Yes. Levothyroxine requirements increase by about 20 to 30% during pregnancy and the increase needs to happen as soon as pregnancy is confirmed, per the 2017 American Thyroid Association pregnancy guidelines[3]. A practical way to make the adjustment is to add 2 extra doses per week (9 doses total instead of 7).
For a new hypothyroidism diagnosis during pregnancy, the starting dose is 2.0 to 2.4 mcg/kg/day[3]. The TSH target is below 2.5 mU/L in the first trimester and below 3.0 in the second and third. TSH should be checked every 4 weeks through the first half of pregnancy, and the dose should be returned to the pre-pregnancy level right after delivery with a TSH recheck at 6 weeks.
For subclinical hypothyroidism the starting dose of levothyroxine is lower than full replacement, usually 25 to 50 mcg per day, or about 1.0 to 1.2 mcg/kg/day if weight-based dosing is used[5]. That’s because the thyroid is still producing some hormone, so the goal is to supplement rather than replace.
Not every patient with subclinical hypothyroidism needs treatment. A TSH above 10 mU/L is the clearest indication to treat. Between 4.5 and 10, the decision depends on positive TPO antibodies, symptoms, pregnancy or trying to conceive, a goiter, or cardiovascular risk factors[5]. If you’re not sure whether to treat, read my full breakdown of subclinical hypothyroidism treatment.
No, 100 mcg of levothyroxine is an average adult dose, not a high dose. The full replacement dose for an average 62 kg (137 lb) adult is exactly 100 mcg per day, based on the 1.6 mcg/kg formula. Most adults on full replacement land somewhere between 75 and 175 mcg per day.
Doses are considered high when they exceed what’s needed to normalize TSH. A 100 mcg dose that keeps your TSH in the normal range isn’t too much. A 100 mcg dose that pushes your TSH below 0.3 (when TSH suppression isn’t the goal) is too much, regardless of whether the number sounds low or high. What matters is how your labs and symptoms respond, not the dose in isolation.
The signs that your levothyroxine dose is too high look like mild hyperthyroidism or thyrotoxicosis: fast or irregular heartbeat, palpitations, anxiety or feeling wired, insomnia, hand tremor, heat intolerance, increased sweating, unintentional weight loss, loose stools or diarrhea, and irritability. A TSH below 0.3 mU/L on labs, sometimes combined with an elevated free T4 or free T3, confirms it.
Long-term overtreatment with levothyroxine carries real risks, not just discomfort. Suppressed TSH over months to years is associated with atrial fibrillation, accelerated bone loss, and osteoporosis, particularly in postmenopausal women and older adults. If you suspect your dose is too high, don’t stop it cold. Ask your prescriber to recheck TSH and adjust, usually by 12.5 to 25 mcg at a time. For a deeper look at the symptoms, read my guide on Synthroid and levothyroxine side effects.
The signs that your levothyroxine dose is too low are the classic symptoms of hypothyroidism persisting or returning: fatigue, brain fog, constipation, weight gain, cold intolerance, dry skin, hair shedding, low mood or depression, muscle aches, menstrual irregularities, and slow heart rate. A TSH above the upper end of the normal range on labs confirms it.
A common pattern I see in practice is a patient whose TSH technically sits within the “normal” range (for example, 3.5 to 4.5 mU/L) but who still has symptoms. That’s often undertreatment. The optimal TSH for most hypothyroid patients is closer to 1.0 to 2.0 mU/L. If you’re still symptomatic at a “normal” TSH, ask your prescriber to check free T3 and reverse T3, and read my guide on optimal TSH levels for thyroid patients. Weight gain that won’t budge even on medication is another common sign of undertreatment, covered in my post on levothyroxine and weight gain.
Take levothyroxine on an empty stomach, either 30 to 60 minutes before breakfast with water, or at bedtime at least 3 to 4 hours after your last meal[1]. Consistency matters more than which of those two windows you pick. Whatever schedule you choose, stick to it every day, because the timing directly affects how much levothyroxine actually gets absorbed.
Separate levothyroxine from calcium supplements, iron supplements, magnesium, biotin, multivitamins containing those minerals, proton pump inhibitors (omeprazole, pantoprazole, esomeprazole), antacids, coffee, and high-fiber foods by at least 4 hours. All of these reduce absorption. Dairy falls into the same category because of its calcium content, and biotin is worth separating because high doses can also interfere with thyroid lab testing. For a complete list of medications and foods that affect levothyroxine absorption, read my guide on foods and medications to avoid with thyroid medication.
Recheck your TSH 6 to 8 weeks after starting or changing a dose of levothyroxine[1]. That’s long enough for your TSH to stabilize on the new dose. Checking earlier gives a misleading number because TSH takes about 6 weeks to fully reflect a change in thyroid hormone levels.
Pregnancy is the exception. TSH should be rechecked every 4 weeks through the first half of pregnancy because your thyroid hormone needs change quickly[3]. Post-thyroidectomy patients and anyone being managed for thyroid cancer should follow the schedule their endocrinologist recommends, which is usually more frequent than the 6 to 8 week standard.
Most calculated levothyroxine doses won’t match an available tablet strength exactly, and that’s normal. The calculator rounds to the closest commercially available tablet (25, 50, 75, 88, 100, 112, 125, 137, 150, 175, or 200 mcg for standard tablets), and that rounded dose is a reasonable starting prescription. Tablets can also be split to get intermediate doses.
Two options give you more precision. The first is a 2-tablet combination, which the calculator flags when combining two strengths lands meaningfully closer to your ideal dose (for example, a 100 mcg plus a 25 mcg to hit 125 mcg exactly). The second is switching to Tirosint-SOL, a liquid levothyroxine that comes in more precise increments (13, 25, 37.5, 50, 62.5, 75, 88, 100, 112, 125, 137, 150, 175, and 200 mcg), giving you the tightest possible dose match.
References
- 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. View on PubMed
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocrine Practice. 2012;18(6):988-1028. View on PubMed
- 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. View on PubMed
- Haugen BR, Alexander EK, Bible KC, et al. 2015 American Thyroid Association Management Guidelines for Adult Patients with Thyroid Nodules and Differentiated Thyroid Cancer. Thyroid. 2016;26(1):1-133. View on PubMed
- Peeters RP. Subclinical Hypothyroidism. New England Journal of Medicine. 2017;376(26):2556-2565. View on PubMed