dromlakhani Determine levothyroxine dosing for central hypothyroidism
STEP 1 — Gather Information
Confirm central hypothyroidism: low serum fT4 with low, normal, or mildly elevated TSH in the setting of pituitary disease. Assess for adrenal insufficiency (morning cortisol, ACTH stimulation test if indeterminate). Record patient weight, age, clinical context (pregnancy, estrogen therapy, GH replacement), and baseline fT4.
STEP 2 — Rule In / Rule Out
Rule out adrenal insufficiency before starting L‑T4. If AI is confirmed or cannot be excluded, initiate empiric glucocorticoid replacement and defer L‑T4 until AI is evaluated; if AI is ruled out, proceed to dose calculation.
STEP 3 — Classify or Stratify
Calculate initial L‑T4 dose: weight (kg) × 1.6 μg/kg/d. Adjust upward for pregnancy, estrogen therapy, or GH replacement; adjust downward for age >60 years or comorbidities that increase overtreatment risk. Target fT4 in the mid‑upper half of the reference range. Use this dose as the starting point for L‑T4 therapy.
STEP 4 — Decide
Prescribe the calculated L‑T4 dose (rounded to nearest 25 μg increment). Schedule serum fT4 re‑evaluation in 6–8 weeks and adjust dose by 12.5–25 μg increments to maintain fT4 within target range.
Clinical Guardrails / Mimics / Pitfalls
Do not use TSH to guide L‑T4 dosing in central hypothyroidism. Avoid L‑T3, thyroid extracts, or other formulations. Ensure adrenal insufficiency is evaluated prior to L‑T4 initiation to prevent adrenal crisis. Monitor for signs of over‑replacement (elevated fT4, tachycardia, weight loss) and under‑replacement (persistent hypothyroid symptoms, low fT4).
Concrete Clinical Example
A 60‑kg, 55‑year‑old woman with confirmed central hypothyroidism, no adrenal insufficiency, not pregnant or on estrogen, and not on GH replacement. Initial dose: 60 × 1.6 = 96 μg/d → round to 100 μg L‑T4 daily. Recheck fT4 at 6 weeks; if fT4 is low‑mid, increase to 112.5 μg; if high‑mid, decrease to 87.5 μg.
Source: Hormonal Replacement in Hypopituitarism in Adults: An Endocrine Society Clinical Practice Guideline, Endocrine Society, 2016, DOI:10.1210/jc.2016-2118
TODO: consider adding scripts/calc.py for the ata-lt4-dosing-ch calculator