Levothyroxine Dosage Calculator

How to use this calculator

What this calculator does

This calculator estimates an adult levothyroxine starting-dose context for primary hypothyroidism. It combines body size with the FT4 pattern and uses more cautious starting ranges when age or cardiovascular disease makes full replacement less appropriate. The weight-based path uses the lower of actual body weight and Devine ideal body weight, mirroring a conservative approach when actual weight exceeds ideal weight. The result is an educational estimate only: it does not diagnose hypothyroidism, select a commercial tablet strength, account for pregnancy or thyroid cancer suppression, or replace a clinician's interpretation of TSH, FT4, symptoms, interacting medicines, and absorption. Current U.S. labeling describes full replacement in many adults as about 1.6 mcg/kg/day and emphasizes individualized titration; see the DailyMed levothyroxine prescribing information.

When to use it

Use the calculator to understand how a starting estimate changes with body size, to compare a low-FT4 overt pattern with a normal-FT4 subclinical pattern, to see why known heart disease leads to a lower initial range, or to prepare questions for a clinician after a new hypothyroidism diagnosis. It is not appropriate for emergencies, myxedema coma, pregnancy-specific dosing, pediatric dosing, thyroid-cancer TSH suppression, or self-adjusting an established prescription.

How to calculate

1. The calculator opens with a ready-to-use demonstration: female, age 30, 60 kg, 165 cm, low FT4, and no heart disease. The first result and a validated example XLSX are available immediately.
2. Replace Sex, Age, Weight, Weight unit, Height, Height unit, Current level of FT4, and Heart disease with the scenario you want to examine. Results update live.
3. Read Estimated starting dose first, then review Dosing weight, Ideal body weight, Weight basis, Thyroid pattern, and TSH target context. The Calculation details table shows the same model through its Item, Value, and Role columns.
4. Select Download Excel to export the current validated model to a real XLSX workbook. Reset clears the demonstration values and all calculated state; Download Excel is then disabled until a complete valid input set is entered again.

Input guide

Sex is required and accepts Female or Male; it affects only the Devine ideal-body-weight constant, so changing sex can change the dosing weight when ideal weight is below actual weight. Age is a required whole number from 18 to 100; 30 is a valid example. Older age can move the starting logic away from full weight-based replacement. Weight is required, must be positive, and accepts decimal-dot notation with conventional comma thousands grouping; 60 kg is the startup example. Do not paste scientific notation or a decimal comma such as “60,5.” Weight unit accepts kg or lb and converts the entered value when switched.

Height is required and supports 120 – 230 cm or the equivalent in inches; 165 cm is the example. It drives the Devine ideal-body-weight estimate. Height unit accepts cm or in and converts the current height on change. Current level of FT4 is required and uses the laboratory's own interpretation: choose Low when FT4 is below that lab's reference range or Normal when it remains in range. A normal FT4 with elevated TSH is the typical laboratory pattern called subclinical hypothyroidism, for which starting doses are often lower. Heart disease is optional; check it for known cardiovascular or coronary disease. This moves the estimate to a cautious low-dose range because thyroid hormone can increase cardiac workload. A common mistake is treating any one field as sufficient to prescribe; the model is intentionally narrower than real clinical decision-making.

Output guide

Estimated starting dose is shown in micrograms per day. It may be a calculated single estimate or a cautious range; it is a starting context, not an exact prescription. Dosing weight is the kilogram value used by the weight-based path. Ideal body weight is the Devine height-and-sex estimate. Weight basis tells you whether actual or ideal weight was the lower value. Thyroid pattern labels low FT4 as overt-pattern hypothyroidism and normal FT4 as a subclinical pattern. TSH target context is a broad age-sensitive educational range; an individual target must follow the treating clinician and laboratory reference interval. In the table, Item names the model component, Value gives the current number or range with units, and Role explains how that row contributes to the result.

Worked example

With the startup values – female, age 30, 60 kg, 165 cm, low FT4, no heart disease – the Devine estimate is about 56.9 kg. Because 56.9 kg is lower than the entered 60 kg, the calculator uses 56.9 kg as the dosing weight. On the younger-adult, low-FT4, no-cardiac-disease path, the educational factor is 1.6 mcg/kg/day. Multiplying 56.9 by 1.6 gives about 91.1 mcg/day, displayed as approximately 91 mcg/day. Actual tablet strength and later dose adjustments depend on clinical response and laboratory testing, not rounding this number to a pill on your own.

Learn more

The American Thyroid Association's thyroid hormone treatment guide explains why levothyroxine is usually taken consistently and why TSH is used to monitor primary hypothyroidism. For practical administration guidance, the NHS levothyroxine medicine page describes taking it on an empty stomach and separating it from factors that can affect absorption.