What initial levothyroxine dose should be started in a patient who was previously on levothyroxine 75 µg, stopped it, and now has a TSH of 120 mIU/L?

Restart at 75 µg levothyroxine immediately

For a patient who was previously on levothyroxine 75 µg, stopped taking it, and now has a TSH of 120 mIU/L, restart at the previous dose of 75 µg daily. This patient can resume their previously employed full replacement dose since the interruption appears to be less than 6 weeks and there is no mention of intercurrent cardiac events or marked weight loss 1.

Clinical Reasoning

Why Resume the Previous Dose?

The guidelines explicitly address this scenario: patients resuming L-thyroxine therapy after interruption (less than 6 weeks) without an intercurrent cardiac event or marked weight loss may resume their previously employed full replacement doses 1. The TSH of 120 mIU/L confirms severe hypothyroidism from medication non-adherence, but the patient's previous stable dose of 75 µg represents their known replacement requirement.

Age-Based Considerations

Critical caveat: If this patient is older than 50-60 years, you must assess for coronary heart disease before restarting at 75 µg:

• Without cardiac disease: Can restart at 75 µg 1
• With known cardiac disease or age >60: Start lower at 50 µg daily, then titrate up to avoid precipitating angina or atrial fibrillation 1

Alternative Approach for Unknown Previous Dose

If the previous dose were unknown, the TSH-based dosing strategy would apply. For TSH >12 mIU/L (and certainly >100 mIU/L), guidelines suggest starting at 75 µg daily 1. A randomized controlled trial demonstrated that TSH-based initial dosing (75 µg for TSH >12 mIU/L) achieved euthyroidism with minimal adjustments after 2 months 1.

Monitoring and Titration

• Recheck TSH in 4-8 weeks after restarting therapy 1
• Adjust dose in 12.5-25 µg increments based on TSH response 1
• Goal TSH: normal reference range (generally 0.5-4.5 mIU/L) for primary hypothyroidism 1

Administration Instructions

Instruct the patient to take levothyroxine:

• 30-60 minutes before breakfast with water, or
• At bedtime, 4 hours after the last meal 1
• Avoid taking with substances that interfere with absorption (calcium, iron, soy products, proton pump inhibitors) 1

Common Pitfalls to Avoid

1. Do not start at a lower "cautious" dose unless the patient has cardiac disease or is elderly—this unnecessarily prolongs hypothyroidism
2. Do not use weight-based dosing (1.6 µg/kg) when a previous stable dose is known—the patient already demonstrated their requirement
3. Do not wait longer than 8 weeks to recheck TSH—earlier assessment prevents prolonged symptomatic hypothyroidism
4. Address medication adherence to prevent recurrence—this TSH elevation suggests complete non-compliance