Adult Hypothyroidism: Starting Levothyroxine — free guideline decision tool

This tool guides starting levothyroxine in adult hypothyroidism by type (overt/subclinical/central), TSH, and cardiac/pregnancy status, including a starting strategy and weight-based dose estimate.

Overt/central → replace (central: assess adrenal function first, monitor FT4). Subclinical: TSH ≥ 10 → treat; TSH < 10 with pregnancy/symptoms/TPOAb/CV risk → consider. Full dose about 1.6 μg/kg/d (adjusted in older/cardiac patients and pregnancy per current guidelines).

Use to decide whether to replace and how to start: overt and central hypothyroidism need replacement, subclinical disease is treated at TSH ≥ 10 or with high-risk features, and starting dose is tailored to age/cardiac status and pregnancy.

In central hypothyroidism, adrenocortical insufficiency must be assessed and corrected before levothyroxine to avoid precipitating adrenal crisis, and monitoring uses FT4 rather than TSH. (original synthesis · not guideline verbatim) Older patients or those with ischemic heart disease start low and uptitrate slowly. Pregnancy requires prompt full replacement with a higher dose and trimester-specific TSH targets.

For licensed clinicians and clinical researchers. Interpret results with history, investigations and local protocols; not a diagnosis or prescription, and not a substitute for multidisciplinary decision-making or local guidelines.

Calculation runs locally in the browser; inputs and results are not uploaded. For licensed clinicians only; interpret with history, investigations and local protocols — not a diagnosis or prescription.

Inputs: Hypothyroidism type Subclinical hypothyroidism (TSH↑, FT4 normal), TSH (for subclinical, mU/L) 8 mU/L, Marked symptoms / positive TPOAb / planning pregnancy / dyslipidemia etc. Present, Age > 50 yr or ischemic heart disease Yes, Pregnancy Yes, Weight (optional, for dose estimate) 60 kg → Result: Disposition Consider LT4 replacement（Decision: Subclinical hypothyroidism, TSH < 10, but with pregnancy/planning pregnancy, marked symptoms, positive TPOAb, or high cardiovascular risk: treatment may be considered., Starting strategy: Pregnancy: start at full replacement dose and reach target promptly; the dose is higher than non-pregnant, adjusted to trimester-specific TSH targets., Dose estimate: Full replacement dose about 1.6 μg/kg/d (titrate by thyroid function; other doses per current guidelines and local protocols)） Inputs: Hypothyroidism type Overt hypothyroidism (TSH↑ + FT4↓), TSH (for subclinical, mU/L) 8 mU/L, Marked symptoms / positive TPOAb / planning pregnancy / dyslipidemia etc. Present, Age > 50 yr or ischemic heart disease Yes, Pregnancy Yes, Weight (optional, for dose estimate) 60 kg → Result: Disposition Recommend LT4 replacement therapy（Decision: Overt hypothyroidism (elevated TSH + low FT4) needs replacement, usually lifelong., Starting strategy: Pregnancy: start at full replacement dose and reach target promptly; the dose is higher than non-pregnant, adjusted to trimester-specific TSH targets., Dose estimate: Full replacement dose about 1.6 μg/kg/d (titrate by thyroid function; other doses per current guidelines and local protocols)）