TSH Levothyroxine
Dose Calculator

Levothyroxine (L-thyroxine, T4) is the treatment of choice for primary hypothyroidism and remains one of the most prescribed medications worldwide. Despite its wide use, getting the dose right requires careful initial estimation followed by regular TSH monitoring and incremental adjustments.

TSH (thyroid-stimulating hormone) is the primary biochemical marker used to guide both initiation and titration. A suppressed or very elevated TSH at baseline provides important context for the starting dose and the urgency of titration.

This calculator estimates the starting dose based on the patient's weight, age, sex, and clinical risk factors (cardiac disease, elderly), then flags safety warnings when specific conditions are present. It also supports a fixed-dose entry mode for clinicians who prefer to start at a set amount and titrate empirically.

The standard approach to levothyroxine initiation uses a weight-based formula as the starting point, with mandatory adjustments for age, cardiac status, and pregnancy.

Enter the patient's weight and optionally their age, sex, and baseline TSH. Select applicable flags for elderly (≥65 years, or clinical frailty) and cardiac disease (ischaemic heart disease, arrhythmia, or heart failure). The calculator applies the appropriate dosing strategy and rounds to the nearest tablet strength.

A pregnancy checkbox appears for female patients. Levothyroxine requirements typically increase by 30–50% in pregnancy and must be adjusted promptly — often before TSH rises above the pregnancy-specific reference range — to avoid fetal neurodevelopmental consequences.

Inline warnings are triggered for: suppressed TSH (suggesting over-replacement or inappropriate treatment), very elevated TSH (indicating severe or prolonged hypothyroidism requiring more urgent assessment), and the cardiac/elderly combination (high-risk titration alert).

A fixed-dose mode overrides the weight-based estimate and allows the clinician to enter any dose directly. This is useful for documenting an empirically chosen starting dose or adjusting from a current maintenance dose.

The rounding step selector allows doses to be snapped to the nearest 12.5, 25, or 50 mcg increment depending on the tablet strengths available locally.

Normal TSH range (typically 0.4–4.0 mU/L) represents adequate thyroid replacement in most adults. The goal of therapy is to maintain TSH within this range while resolving symptoms. Some guidelines target the lower half of the reference range (0.5–2.0 mU/L) for symptomatic relief, though this remains debated.

Suppressed TSH (<0.1 mU/L) during levothyroxine therapy indicates over-replacement or, in certain cancers, intentional suppression therapy. Chronic TSH suppression is associated with atrial fibrillation and osteoporosis and should be avoided in standard hypothyroid management.

TSH >10 mU/L at initiation indicates overt hypothyroidism requiring definitive treatment. Very high baseline TSH (>100 mU/L) may reflect prolonged hypothyroidism or myxoedema and warrants specialist input before initiating replacement.

Monitoring schedule: TSH should be checked 4–6 weeks after each dose change. Once stable, annual monitoring is appropriate for most patients. Pregnancy, new medications (particularly those affecting absorption), and new gastrointestinal conditions warrant earlier re-checking.

Take levothyroxine on an empty stomach, ideally 30–60 minutes before breakfast, with a full glass of water. Taking it with food, coffee, or dairy products significantly reduces absorption.

Important drug interactions reduce levothyroxine absorption: calcium carbonate, ferrous sulfate, antacids (especially aluminium-containing), cholestyramine, proton pump inhibitors, and many others. Space administration of these agents by at least 4 hours.

Consistency of brand or formulation matters. Different brands of levothyroxine can have slightly different bioavailability. Where possible, patients should take the same brand at each refill — changes in brand should be followed by a TSH recheck in 4–6 weeks.