Levothyroxine Dosage Calculator
Calculate levothyroxine starting dose by weight, age, and FT4 level. Free thyroid medication dosing calculator using AACE/ATA guidelines with tablet strength matching and charts.
About This Calculator
The Levothyroxine Dosage Calculator helps healthcare professionals and patients determine the appropriate starting dose of levothyroxine (synthetic thyroxine/T4) for treating hypothyroidism. Based on the American Association of Clinical Endocrinologists (AACE) and American Thyroid Association (ATA) clinical practice guidelines, the calculator uses weight-based dosing with adjustments for age, disease severity, heart disease status, and treatment goals.
Levothyroxine is the standard of care for hypothyroidism, a condition affecting approximately 5-10% of the global population, with higher prevalence in women and older adults. The starting dose is typically 1.6 mcg per kilogram of dosing weight per day, calculated using the ideal body weight from the Devine formula. Adjustments are made for patients over age 50, those with subclinical hypothyroidism (normal FT4 but elevated TSH), heart disease patients who require a very low starting dose, and thyroid cancer patients who need TSH suppression at higher doses.
How the Dose is Calculated
The calculator first determines the ideal body weight (IBW) using the Devine formula, which varies by sex. If the patient's actual weight is below IBW, actual weight is used to avoid overdosing. For obese patients (BMI ≥ 30), an adjusted body weight is calculated: IBW + 0.4 × (actual weight − IBW). The base dose of 1.6 mcg/kg/day is then adjusted downward for older age (reduced metabolism of T4), subclinical hypothyroidism (less severe deficiency), and heart disease (risk of arrhythmia with rapid correction), or upward for TSH suppression therapy in thyroid cancer (2.0 mcg/kg/day).
Regional Notes
India: Hypothyroidism is highly prevalent in India, estimated at 10-13% of the population. Common levothyroxine brands include Thyronorm (Abbott), Eltroxin (GSK), and Thyrox (Macleods). The National Rural Health Mission includes thyroid screening in some programs. Annual TSH monitoring costs approximately ₹300-500.
United States: Levothyroxine is one of the most prescribed medications, with over 120 million prescriptions annually. Common brands include Synthroid (AbbVie), Levoxyl (Pfizer), Unithroid (Stevens Jelly), and Tirosint (Akrimax). Generic levothyroxine is widely available and costs approximately $10-20 per month. The AACE/ATA guidelines recommend starting dose of 1.6 mcg/kg and TSH targets of 0.5-2.5 mU/L.
United Kingdom: NICE guidelines recommend levothyroxine for overt hypothyroidism with dose based on weight (1.6 mcg/kg). Common brands include Eltroxin and Levothyroxine Sodium (various generics). The NHS spends approximately £30 million annually on levothyroxine prescriptions. TSH targets follow AACE/ATA guidance but some CCGs recommend a slightly wider range of 0.5-3.0 mU/L.
Frequently Asked Questions
How is levothyroxine starting dose calculated?
Levothyroxine starting dose is calculated using weight-based dosing at 1.6 mcg per kg of dosing weight per day for standard replacement. The dosing weight is determined using the Devine formula for ideal body weight. The dose is adjusted downward for patients over 50 years, subclinical hypothyroidism, and heart disease. For thyroid cancer suppression, a higher dose of 2.0 mcg/kg/day is used.
What is the Devine formula for ideal body weight?
The Devine formula for ideal body weight is: For women — 45.5 kg plus 2.3 kg for every inch over 5 feet (60 inches). For men — 50.0 kg plus 2.3 kg for every inch over 5 feet. Height must first be converted from centimeters to inches by dividing by 2.54. This formula was originally developed for aminophylline dosing but is widely used for levothyroxine dosing as well.
What is a normal starting dose of levothyroxine for hypothyroidism?
For a typical 70 kg adult with overt hypothyroidism, the standard starting dose of levothyroxine is approximately 100-125 mcg per day (1.6 mcg/kg). Older patients (age 50-60) may start at 50-100 mcg, and patients over 60 may start at 25-50 mcg. Patients with heart disease should start at 12.5-25 mcg and titrate slowly. Subclinical hypothyroidism is treated with 25-50 mcg daily.
What levothyroxine tablet strengths are available?
Levothyroxine is available in multiple tablet strengths: 25 mcg, 50 mcg, 75 mcg, 88 mcg, 100 mcg, 112 mcg, 125 mcg, 137 mcg, 150 mcg, 175 mcg, and 200 mcg. The calculator suggests the closest single tablet strength and the best two-tablet combination to match the calculated daily dose. Patients should always use the exact prescribed strength and not split tablets unless directed.
How should levothyroxine be taken for best absorption?
Levothyroxine should be taken on an empty stomach, at least 30-60 minutes before breakfast, with a full glass of water. Avoid taking it with iron supplements, calcium supplements, antacids, or high-fiber foods within 4 hours of the dose, as these can significantly reduce absorption. Taking it consistently at the same time each day is important for stable thyroid hormone levels.
What is the target TSH level for levothyroxine therapy?
For standard replacement therapy in overt hypothyroidism, the target TSH is 0.5-2.5 mU/L. For subclinical hypothyroidism, the target is 0.5-3.0 mU/L. For thyroid cancer patients on TSH suppression therapy, a lower target of 0.1-0.5 mU/L is used. TSH should be rechecked 6-8 weeks after initiating or changing levothyroxine dose, and once stable, annually thereafter.
Is levothyroxine dosing different in India, US, and UK?
The weight-based levothyroxine dosing formula is the same worldwide. However, brand names and availability differ: In India, common brands include Thyronorm, Eltroxin, and Thyrox. In the US, brands include Synthroid, Levoxyl, Unithroid, and Tirosint (gel capsule). In the UK, brands include Eltroxin and Levothyroxine. Bioequivalence between brands can vary, so patients should use the same brand consistently.
When should thyroid function be rechecked after starting levothyroxine?
TSH should be measured 6-8 weeks after initiating levothyroxine therapy or after any dose adjustment. Once a stable dose is achieved, TSH is typically monitored every 6-12 months. More frequent monitoring is needed during pregnancy, after significant weight changes, when starting or stopping medications that affect thyroid function (estrogen, androgens, certain antidepressants), or if symptoms of hypo- or hyperthyroidism develop.