How should I adjust my levothyroxine dose given a TSH of 34 mIU/L, free thyroxine of 4.3 µg/dL, and normal triiodothyronine while taking 100 µg levothyroxine daily?

Increase Levothyroxine Dose Immediately

Your TSH of 34 mIU/L indicates severe undertreatment requiring immediate dose escalation of 25-50 mcg above your current 100 mcg daily dose. 1

Why Your Current Dose Is Inadequate

• A TSH >10 mIU/L carries approximately 5% annual risk of progression to overt hypothyroidism and is associated with cardiac dysfunction, elevated LDL cholesterol, and significant quality of life impairment 1
• Your TSH of 34 mIU/L represents severe primary hypothyroidism despite levothyroxine therapy, indicating your current 100 mcg dose provides insufficient thyroid hormone replacement 1
• The normal T3 you mention is expected—T3 remains normal longer than T4 in undertreated hypothyroidism, so normal T3 does NOT indicate adequate treatment when TSH is this elevated 2, 3

Recommended Dose Adjustment

Increase levothyroxine by 25-50 mcg immediately:

• If you are under 70 years without cardiac disease: Increase to 125-150 mcg daily (25-50 mcg increment) 1, 4
• If you are over 70 years OR have cardiac disease: Increase to 125 mcg daily (25 mcg increment only) and titrate more cautiously 1, 4

The FDA-approved dosing guideline recommends 5-25 mcg increments every 4-6 weeks for adults at cardiac risk, but given your severely elevated TSH, a 25-50 mcg increase is appropriate for most patients 4

Monitoring Protocol

• Recheck TSH and free T4 in 6-8 weeks after dose adjustment 1, 4
• Target TSH: 0.5-4.5 mIU/L with normal free T4 1
• Continue adjusting by 12.5-25 mcg increments every 6-8 weeks until TSH normalizes 1
• Once stable, monitor TSH every 6-12 months 1

Critical Safety Considerations

Before increasing your dose, your physician should:

• Rule out concurrent adrenal insufficiency (check morning cortisol if you have unexplained hypotension, hyponatremia, or salt craving), as starting or increasing thyroid hormone before treating adrenal insufficiency can precipitate life-threatening adrenal crisis 1
• If you have cardiac disease, obtain an ECG to screen for arrhythmias before dose escalation 1

Why Normal T3 Doesn't Mean You're Adequately Treated

• In hypothyroid patients on levothyroxine, the body preferentially converts available T4 to T3 to maintain normal T3 levels even when T4 and TSH are abnormal 2, 3
• Studies show that achieving normal TSH on levothyroxine monotherapy often results in lower free T3 levels than in euthyroid individuals, despite normal total T3 2, 5, 3
• Your normal T3 simply reflects your body's compensatory mechanism—it does NOT indicate your dose is adequate when TSH is 34 mIU/L 2, 3

Common Pitfalls to Avoid

• Never delay treatment based on normal T3 alone—TSH is the primary marker for dose adequacy in primary hypothyroidism 1
• Do not make small 12.5 mcg adjustments when TSH is this severely elevated—you need 25-50 mcg increases to normalize thyroid function efficiently 1
• Do not recheck TSH before 6-8 weeks—levothyroxine requires this interval to reach steady state, and earlier testing leads to inappropriate dose adjustments 1, 4
• Approximately 25% of patients on levothyroxine are unintentionally maintained on insufficient doses, leading to persistent hypothyroid symptoms and cardiovascular dysfunction 1

What to Expect After Dose Increase

• Symptoms (fatigue, weight gain, cold intolerance, constipation) should improve within 6-8 weeks of adequate replacement 1
• Cardiac function, lipid profile, and quality of life will improve as TSH normalizes 1
• If TSH remains >10 mIU/L after dose adjustment, continue increasing by 12.5-25 mcg every 6-8 weeks until normalized 1