Endocrine

Levothyroxine Initial Dose (weight-based)

Estimates the initial levothyroxine dose for primary hypothyroidism: full weight-based replacement or low-dose initiation in elderly/cardiac patients.

Enter the required values to see the result.

Formula

\text{Full replacement dose} \approx 1.6 \; \mu\text{g} \times \text{weight (kg)} \; \text{per day}

Caveats & limitations

Take fasting, 30–60 minutes before food or caffeine; separate from calcium, iron and PPIs.
Requirement rises with weight gain, pregnancy and malabsorption; reassess TSH 6–8 weeks after any change.
Central hypothyroidism is dosed against free T4, not TSH.
Actual requirement varies widely — the estimate is a starting point, not a target.

Evidence & UK guidance

Primary evidence

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.
PubMed DOI

UK practice

NICE NG145: consider levothyroxine 1.6 micrograms/kg/day (rounded to the nearest 25 µg) for adults under 65 with primary hypothyroidism and no cardiovascular disease; for adults ≥65 or with cardiovascular disease, start 25–50 µg/day and titrate. Recheck TSH 6–8 weeks after any change. Pregnancy requires urgent specialist input.

Official resources

Formula and citations last verified: 2026-08-03

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