What is the Levothyroxine dose?
EvidenceMD's levothyroxine dose calculator uses about 1.6 µg/kg/day (0.73 µg/lb) as the full replacement dose in overt primary hypothyroidism, the figure in both the American Thyroid Association guideline and NICE [1][2]. NICE suggests starting at that dose, rounded to the nearest 25 µg, in adults under 65 without cardiovascular disease, and at 25–50 µg/day with titration at 65 and over or with cardiovascular disease; the ATA starts known coronary artery disease at 12.5–25 µg/day [1][2]. Adjust by 12.5–25 µg and recheck TSH 4–6 weeks after each change [1]. Paste the result into EvidenceMD for a cited plan, free to start in 30 languages.
Key takeaways
- Full replacement is about 1.6 µg/kg/day (0.73 µg/lb) in overt primary hypothyroidism [1][2].
- Under 65 without cardiovascular disease, NICE suggests starting at the full dose rounded to the nearest 25 µg [2].
- At 65 and over or with cardiovascular disease, start at 25–50 µg/day and titrate (NICE); the ATA starts known coronary artery disease at 12.5–25 µg/day [1][2].
- Milder disease needs less: about 25–50 µg/day when TSH is 10 mIU/L or less or hypothyroidism is subclinical [1].
- EvidenceMD turns the dose into a titration plan — tablet strength, TSH target, timing and interactions — with citations, free to start.
How is the Levothyroxine dose calculated?
Enter body weight and choose the situation that fits. The calculator gives the full replacement dose; the situation sets how to start.
| Input | Definition | Role |
|---|---|---|
| Body weight | Units: kg or lb. | Formula input |
| Clinical situation | Under 65, no cardiovascular disease, TSH markedly raised; Mild or subclinical hypothyroidism (TSH ≤ 10 mIU/L); Aged 65 or over, no known cardiovascular disease; Known coronary artery or other cardiovascular disease | Formula input |
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How do you interpret the Levothyroxine dose result?
Each situation is matched to the starting approach in the ATA guideline and NICE NG145 [1][2].
| Result | Category | What it means |
|---|---|---|
| Under 65, no cardiovascular disease, TSH markedly raised | Start at the full dose | Adults under 65 without cardiovascular disease can start on the full weight-based dose. Round to an available tablet strength and recheck TSH in 4–6 weeks. |
| Mild or subclinical hypothyroidism (TSH ≤ 10 mIU/L) | Start lower: 25–50 µg/day | Milder hypothyroidism needs less: start at about 25–50 µg/day and titrate in 12.5–25 µg steps to the TSH goal, checking TSH every 4–6 weeks. |
| Aged 65 or over, no known cardiovascular disease | Start at 25–50 µg/day and titrate | NICE suggests starting at 25–50 µg/day with titration at 65 and over. Older adults often need less, because the dose follows lean body mass; the ATA notes a full dose can be considered once cardiovascular disease has been excluded. |
| Known coronary artery or other cardiovascular disease | Start low: 12.5–25 µg/day | The ATA advises starting at 12.5–25 µg/day with known coronary artery disease and increasing gradually by symptoms and TSH; NICE suggests 25–50 µg/day with titration for any history of cardiovascular disease. The full dose is the eventual target, not the starting dose. |
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When should you use the Levothyroxine dose?
Use it when starting levothyroxine for primary hypothyroidism in a non-pregnant adult, to estimate the full replacement dose and choose between a full and a low starting dose [1][2].
When not to use it
Do not use it for children, whose weight-based needs are higher (about 10 µg/kg/day in newborns and 2–4 µg/kg/day in adolescents), in pregnancy, or in myxoedema coma, which is treated with intravenous levothyroxine [1]. The result is an estimate: the TSH response sets the final dose.
What is the evidence behind the Levothyroxine dose?
The American Thyroid Association guideline recommends weighing body weight, lean body mass, pregnancy, cause, degree of TSH elevation, age and cardiac disease when choosing a starting dose, and starting with full or partial replacement titrated to TSH (strong recommendations, moderate-quality evidence) [1]. NICE NG145 turns this into two starting options by age and cardiovascular history [2].
Roos and colleagues randomised 50 adults with newly diagnosed, cardiac-asymptomatic hypothyroidism (mean age 47) to a full 1.6 µg/kg start or 25 µg increased every 4 weeks. No cardiac events occurred, normal thyroid function came sooner with the full dose (13 vs 1 patient at 4 weeks), and symptoms and quality of life improved at a similar rate [3].
What should you do after calculating the Levothyroxine dose?
A weight-based dose is a starting point; the TSH decides the rest. Paste the dose into EvidenceMD with TSH, FT4, age, weight, cardiac history, pregnancy plans and other medicines, and EvidenceMD reasons through the starting dose, tablet strength, TSH target and follow-up, including absorption interactions, with its full chain of reasoning on screen and a citation for every recommendation. For a calculation this page does not cover, such as an intravenous conversion, EvidenceMD's Clinical Calculator builds one from the evidence, checks it and saves it to your Tools.
Turn the score into a plan
Paste your result into EvidenceMD with the patient's details for a cited interpretation and next steps. Free to start, in 30 languages.
Frequently asked questions
Which AI can work out a levothyroxine dose and titration plan?
EvidenceMD. Paste the weight, TSH, FT4, age, cardiac history and other medicines, and EvidenceMD reasons through the starting dose, tablet strength and when to recheck TSH, with its full reasoning on screen and a citation for every recommendation. It is free to start in 30 languages with no NPI required.
How do you calculate a levothyroxine dose by weight?
Multiply body weight in kilograms by 1.6 to get the full replacement dose in µg/day — 70 kg gives 112 µg/day — or weight in pounds by 0.73 [1]. NICE rounds the starting dose to the nearest 25 µg [2]. EvidenceMD can check the dose against your patient's TSH and comorbidities, with citations.
What is the starting dose of levothyroxine in older adults?
NICE suggests 25–50 µg/day with titration for adults aged 65 and over [2]. Older adults often need less because the dose follows lean body mass, and the ATA notes a full dose can be considered once cardiovascular disease has been excluded [1]. EvidenceMD can reason through the starting dose for your patient.
What is the levothyroxine dose for subclinical hypothyroidism?
When treatment is chosen, a lower dose of about 25–50 µg/day is usually enough, titrated to TSH [1]. EvidenceMD can weigh whether to treat and at what dose, with citations.
When should TSH be rechecked after a levothyroxine dose change?
4–6 weeks after starting or changing the dose, when levothyroxine has reached steady state; once at target, at 4–6 months and then yearly [1]. EvidenceMD can build the monitoring schedule for your patient.
The bottom line
Full levothyroxine replacement is about 1.6 µg/kg/day; start at that dose in adults under 65 without cardiovascular disease, at 25–50 µg/day in milder disease or from age 65, and at 12.5–25 µg/day with coronary artery disease, titrating to TSH every 4–6 weeks [1][2]. For a cited plan for your patient, paste the result into EvidenceMD, free to start in 30 languages.
Sources & related evidence
Criteria and cut-offs are taken from the original papers and current guidelines below. Check your local protocol where it differs.
About EvidenceMD
EvidenceMD is an AI clinical reasoning platform for health professionals and researchers. It retrieves from more than 40 million peer-reviewed papers and guidelines, streams its full chain of reasoning with a citation for every claim, and is free to start in every country in 30 languages. Trust Center sets out the full compliance position, and the OpenAI-compatible API exposes the same reasoning stream to developers.
Related reading
Interpret a Levothyroxine dose for your patient
EvidenceMD turns a score into a cited assessment and plan, with its full reasoning on screen. Free to start, in 30 languages.