Half-Life of Levothyroxine (Thyronorm)
The half-life of levothyroxine is approximately 6-8 weeks (approximately 7 days), which is why dose adjustments should only be made after waiting 6-8 weeks to reach steady state, and TSH monitoring should occur at these intervals during titration. 1
Pharmacokinetic Properties
- Levothyroxine has a plasma half-life of approximately 7 days in patients with normal thyroid function 2
- This long half-life allows for once-daily dosing to maintain stable thyroid hormone levels throughout the day 1
- The extended half-life means that steady-state concentrations are not achieved until 6-8 weeks after initiating therapy or changing doses 1
Clinical Implications of the Long Half-Life
Dose Adjustment Timing
- Wait a minimum of 6-8 weeks between dose adjustments to allow levothyroxine to reach steady state before assessing therapeutic response 1
- Adjusting doses too frequently before reaching steady state is a common pitfall that can lead to overtreatment or undertreatment 1
- TSH levels should be rechecked 6-8 weeks after any dose change to properly evaluate the effect 1
Monitoring Schedule
- During initial titration, monitor TSH every 6-8 weeks until the target range is achieved 1
- Once stable on an appropriate maintenance dose, TSH monitoring can be reduced to every 6-12 months or when symptoms change 1
- For patients with cardiac disease or atrial fibrillation, more frequent monitoring may be warranted, with testing repeated within 2 weeks of dose adjustment 1
Overdose Considerations
- In cases of massive levothyroxine overdose, the average half-life of total T4 under standard medical treatment is approximately 106.5 ± 44.6 hours (about 4.4 days) 3
- Extractive procedures like plasmapheresis can shorten this to approximately 59.7 ± 20.2 hours (about 2.5 days) in overdose situations 3
Contrast with Other Thyroid Hormones
- The plasma half-life of full-length parathyroid hormone (PTH) is much shorter at only 2-4 minutes, demonstrating that levothyroxine's week-long half-life is exceptionally long for a hormone replacement therapy 4
- This extended half-life of levothyroxine compared to endogenous thyroid hormone contributes to its suitability as a once-daily replacement therapy 1