What is the initial management for a patient with central hypothyroidism?

Initial Management of Central Hypothyroidism

Start levothyroxine at 1 mcg/kg/day immediately after confirming the diagnosis and ruling out adrenal insufficiency. 1, 2

Critical First Step: Rule Out Adrenal Insufficiency

• ALWAYS evaluate for concurrent adrenal insufficiency BEFORE starting thyroid hormone replacement - this is the most critical pitfall to avoid 1, 2
• Check morning cortisol and ACTH levels to assess the hypothalamic-pituitary-adrenal axis 1, 3
• If adrenal insufficiency is present, start physiologic corticosteroid replacement (hydrocortisone ~10 mg/m² divided as 15 mg morning, 5 mg at 3 pm) FIRST, then initiate thyroid hormone 1, 2
• Starting levothyroxine before corticosteroids can precipitate life-threatening adrenal crisis 2

Confirm the Diagnosis

• Central hypothyroidism presents with low or inappropriately normal TSH with low free T4 - this distinguishes it from primary hypothyroidism which has elevated TSH 1, 2, 4
• Measure both TSH and free T4 to confirm the diagnosis 2, 5
• Evaluate other pituitary hormones since central hypothyroidism often indicates broader pituitary dysfunction 3

Initial Levothyroxine Dosing

For Patients <70 Years Without Cardiac Disease:

• Start levothyroxine at 1 mcg/kg/day (or 1.6 mcg/kg/day based on ideal body weight for primary hypothyroidism dosing) 1, 2, 6
• Young, healthy patients can typically start at full replacement dose 1

For Patients >70 Years OR With Cardiovascular Disease:

• Start with a lower dose of 25-50 mcg/day and titrate gradually 1, 2
• This prevents exacerbation of cardiac symptoms including angina and atrial fibrillation 1, 2, 6
• Titrate more slowly in this population (every 6-8 weeks) 1, 6

Monitoring Strategy - Key Difference from Primary Hypothyroidism

• TSH is NOT reliable for monitoring central hypothyroidism - this is a critical distinction from primary hypothyroidism 2, 6, 3
• Target free T4 in the upper half of the normal reference range - this is your primary monitoring parameter 2, 6, 3, 4
• Check free T4 (and free T3 if available) 6-8 weeks after starting treatment or changing dose 1, 5, 7
• Also monitor free T3 levels as some patients may have normal free T4 but low free T3, indicating inadequate conversion 7

Dose Titration

• Adjust levothyroxine dose by 12.5-25 mcg increments based on free T4 levels 6
• Wait at least 6-8 weeks between dose adjustments due to levothyroxine's long half-life 5, 6
• Continue titrating until free T4 reaches the upper half of normal range and patient is clinically euthyroid 2, 6, 3

Long-Term Monitoring

• Once stable, monitor free T4 levels every 3 months in the first year, then every 6 months thereafter 1
• Assess clinical response including resolution of hypothyroid symptoms 4, 7
• Consider checking biochemical markers of thyroid hormone action (cholesterol, soluble IL-2 receptor) to detect subtle over- or under-treatment 7

Administration Instructions

• Take levothyroxine on an empty stomach, 30-60 minutes before breakfast with full glass of water 6
• Separate from medications that interfere with absorption (iron, calcium, proton pump inhibitors) by at least 4 hours 6
• Emphasize that this is typically lifelong therapy requiring regular monitoring 2

Common Pitfalls to Avoid

• Using TSH to guide therapy - TSH remains low/normal in central hypothyroidism even when adequately treated; rely on free T4 instead 2, 6, 3
• Starting thyroid hormone before addressing adrenal insufficiency - always check cortisol first and replace if needed 1, 2
• Using too high a starting dose in elderly or cardiac patients - start low (25-50 mcg) and titrate slowly 1, 2, 6
• Inadequate monitoring frequency - check free T4 at 6-8 weeks, not sooner, as peak effect takes 4-6 weeks 6