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 ~1.6 μg/kg/d (~1.0 if elderly/cardiac; +30–50% in pregnancy).

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 (25–50, or 12.5 μg/d) and uptitrate slowly. Pregnancy requires prompt full replacement with ~30–50% 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.

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; replacement dose is ~30%–50% higher than non-pregnant, adjusted to trimester-specific TSH targets., Dose estimate: Full replacement dose ~ 134 μg/d (elderly/heart disease ~1.0 μg/kg; pregnancy +30–50%; titrate by thyroid function)） Inputs: Hypothyroidism type Central hypothyroidism (FT4↓, TSH not high), TSH (for subclinical, mU/L) 8 mU/L, Marked symptoms / positive TPOAb / planning pregnancy / dyslipidemia etc. Absent, Age > 50 yr or ischemic heart disease No, Pregnancy No, Weight (optional, for dose estimate) 60 kg → Result: Disposition Recommend LT4 replacement (first assess adrenocortical function)（Decision: Central hypothyroidism needs replacement, but before starting LT4 first assess and correct any adrenocortical insufficiency to avoid precipitating crisis; monitor central hypothyroidism by FT4, not TSH., Starting strategy: Young, no heart disease: may start at full replacement dose (~1.6 μg/kg/d)., Dose estimate: Full replacement dose ~ 96 μg/d (~1.6 μg/kg; pregnancy +30–50%; titrate by thyroid function)）