Thyroid Hormone Dose Calculator

Calculate levothyroxine dose adjustment from TSH levels and body weight. Enter values for instant results with step-by-step formulas.

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Formula

Full replacement dose = Body weight (kg) x 1.6 mcg/kg/day

The standard levothyroxine replacement dose for primary hypothyroidism is 1.6 mcg/kg/day based on ideal body weight. This is adjusted for age (elderly: 70% of full dose), cardiac status (cardiac disease: 40% of full dose), and indication (cancer suppression: 2.0-2.5 mcg/kg/day). Dose is rounded to the nearest available tablet strength.

Worked Examples

Example 1: New Diagnosis of Primary Hypothyroidism

Problem: A healthy 35-year-old woman weighing 65 kg is newly diagnosed with primary hypothyroidism with TSH of 45 mIU/L. Calculate the appropriate starting dose of levothyroxine.

Solution: Full replacement dose calculation:\nDose = Weight x 1.6 mcg/kg/day\nDose = 65 kg x 1.6 = 104 mcg/day\nNearest tablet strength: 100 mcg\nAge adjustment: None (young, healthy adult)\nStarting approach: Can start at full replacement dose\nRecheck TSH in 6-8 weeks\nTarget TSH: 0.5-4.0 mIU/L

Result: Recommended dose: 100 mcg levothyroxine daily - Recheck TSH in 6-8 weeks

Example 2: Dose Adjustment in Elderly Patient

Problem: A 78-year-old man weighing 75 kg with coronary artery disease is on levothyroxine 75 mcg daily with TSH of 12 mIU/L. Calculate dose adjustment.

Solution: Current dose: 75 mcg/day\nTSH significantly elevated (>10): Increase by 25 mcg\nSuggested new dose: 100 mcg/day\nAge/cardiac consideration: Elderly with CAD\nPreferred approach: Increase by 12.5-25 mcg (conservative)\nSuggested: 88 mcg or 100 mcg tablet\nFull replacement (elderly-adjusted): 75 x 0.7 = 52.5 mcg\nRecheck TSH in 6-8 weeks with cardiac monitoring

Result: Increase to 88-100 mcg daily with cardiac monitoring - Recheck TSH in 6-8 weeks

Frequently Asked Questions

How is the levothyroxine dose calculated for hypothyroidism?

The standard full replacement dose of levothyroxine for primary hypothyroidism is approximately 1.6 micrograms per kilogram of ideal body weight per day. This calculation provides a starting point that is then individualized based on TSH response. For a 70 kg patient, the estimated full replacement dose is approximately 112 mcg daily. However, many patients do not require the full calculated dose, and starting at a lower dose with gradual titration is often preferred. The actual dose needed varies based on the degree of residual thyroid function, body composition (lean mass versus fat mass), age, medications affecting absorption or metabolism, and the specific levothyroxine formulation used. TSH should be rechecked 6-8 weeks after any dose change.

What TSH target should be used for thyroid cancer suppression therapy?

TSH suppression targets for differentiated thyroid cancer vary based on the risk stratification of the individual patient, as established by the American Thyroid Association guidelines. For high-risk patients (large tumors, aggressive histology, extrathyroidal extension, distant metastases), the initial TSH target is below 0.1 mIU/L, which typically requires doses of 2.0-2.5 mcg/kg/day. For intermediate-risk patients, the target is 0.1-0.5 mIU/L. For low-risk patients who have had excellent response to initial therapy, the target TSH is 0.5-2.0 mIU/L, which is near the normal range. Over time, if surveillance shows no evidence of recurrence, TSH suppression targets are relaxed to minimize the risks of prolonged thyroid hormone excess, including atrial fibrillation and osteoporosis.

How do I get the most accurate result?

Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.

Can I use Thyroid Hormone Dose Calculator on a mobile device?

Yes. All calculators on NovaCalculator are fully responsive and work on smartphones, tablets, and desktops. The layout adapts automatically to your screen size.

Is Thyroid Hormone Dose Calculator free to use?

Yes, completely free with no sign-up required. All calculators on NovaCalculator are free to use without registration, subscription, or payment.

How do I interpret the result?

Results are displayed with a label and unit to help you understand the output. Many calculators include a short explanation or classification below the result (for example, a BMI category or risk level). Refer to the worked examples section on this page for real-world context.