Levothyroxine Dose Calculator

What Is Levothyroxine?

Levothyroxine (L-T4) is a synthetic form of thyroxine (T4), the primary hormone produced by the thyroid gland. It is the standard of care for the treatment of hypothyroidism and is one of the most commonly prescribed medications worldwide. Levothyroxine replaces or supplements endogenous thyroid hormone, restoring normal metabolic function, energy levels, and physiological homeostasis.

Indications for Levothyroxine Therapy

Primary hypothyroidism is the most common indication, resulting from thyroid gland failure due to Hashimoto's thyroiditis, thyroidectomy, radioactive iodine ablation, or other causes. The full replacement dose is approximately 1.6 mcg/kg/day of ideal body weight. TSH suppression therapy is used in differentiated thyroid cancer to reduce the risk of recurrence, requiring higher doses of 2.0–2.5 mcg/kg/day to suppress TSH below 0.1 mU/L. Subclinical hypothyroidism (elevated TSH with normal free T4) may warrant treatment starting at 25–50 mcg/day depending on symptoms, antibody status, and cardiovascular risk. Myxedema coma is a life-threatening emergency requiring IV levothyroxine loading doses of 200–400 mcg.

Weight-Based Dosing

The cornerstone of levothyroxine dosing is body weight. In healthy adults under 50 without cardiac disease, the full replacement dose of 1.6 mcg/kg/day can often be initiated immediately. For a 70 kg patient, this equals approximately 112 mcg/day. In obese patients, dosing should be based on lean body weight or ideal body weight, as adipose tissue does not significantly increase T4 requirements. Using total body weight in obese patients leads to overtreatment.

Age-Adjusted Dosing

Elderly patients and those with known or suspected coronary artery disease require cautious initiation. The recommended starting dose is 12.5–25 mcg/day, with increases of 12.5–25 mcg every 4–6 weeks. Rapid thyroid hormone replacement in elderly patients can precipitate angina pectoris, myocardial infarction, or cardiac arrhythmias. Pediatric dosing is higher per kilogram than adults due to the increased metabolic demands of growth, ranging from 10–15 mcg/kg/day in neonates to 2–3 mcg/kg/day in adolescents.

Pregnancy Considerations

Thyroid hormone requirements increase during pregnancy by approximately 25–50%, driven by increased thyroxine-binding globulin (TBG), expanded plasma volume, and placental deiodination. The ATA recommends that women on levothyroxine increase their dose by approximately 30% (e.g., 2 extra tablets per week of their current dose) as soon as pregnancy is confirmed. TSH should be monitored every 4 weeks during the first half of pregnancy and at least once per trimester thereafter. The target TSH in pregnancy is trimester-specific, generally < 2.5 mU/L in the first trimester.

Available Tablet Strengths

Levothyroxine is available in a wide range of oral tablet strengths: 25, 50, 75, 88, 100, 112, 125, 137, 150, 175, 200, and 300 mcg. This granularity allows precise dose titration, which is important given that even small changes (12.5–25 mcg) can significantly affect TSH levels. Tablets should not be split, as content uniformity may be affected. Liquid and softgel capsule formulations are also available and may improve absorption consistency in patients with GI disorders or medication interactions.

Key References

• Jonklaas J, et al. Guidelines for the Treatment of Hypothyroidism: Prepared by the ATA Task Force. Thyroid. 2014;24(12):1670–1751.
• Garber JR, et al. Clinical Practice Guidelines for Hypothyroidism in Adults. Endocr Pract. 2012;18(6):988–1028.
• Alexander EK, et al. 2017 Guidelines of the ATA for the Diagnosis and Management of Thyroid Disease During Pregnancy. Thyroid. 2017;27(3):315–389.
• Biondi B, Wartofsky L. Treatment with thyroid hormone. Endocr Rev. 2014;35(3):433–512.