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How long does levothyroxine stay in your system?

Medically reviewed by Sally Chao, MD. Last updated on Oct 7, 2021.

Official answer


Levothyroxine will stay in your system for around 4 to 6 weeks. It takes longer for people with hypothyroidism to eliminate levothyroxine (closer to 6 weeks) than those with normal thyroid function.

Levothyroxine contains a synthetic version of T4, one of two hormones produced by the thyroid. Once T4 is in the body, some of it is converted to T3 (the other thyroid hormone), which is why patients with hypothyroidism generally only need to take levothyroxine alone to maintain healthy levels of both thyroid hormones.

The half-life of T4 is around 6 to 7 days, which is used to estimate how long a drug stays in your system. The half-life is the time it takes for the drug in your blood to reduce to half its original value. In general, it takes 5.5 half-lives to eliminate a drug from your body.
6 Days X 5.5 = 33 days
7 Days X 5.5 = 38 days

Levothyroxine will stay in your system approximately 33 - 38 days with personal variation.

However, the time it takes for levothyroxine to be eliminated from your body can vary depending on several factors besides thyroid function, including:

  • Age
  • Weight
  • Overall health
  • Medical conditions
  • Other medications

For example, research suggests that levothyroxine can remain in the body of people over 80 years old for around 7 weeks.

  1. Colucci P, Yue CS, Ducharme M, Benvenga S. A Review of the Pharmacokinetics of Levothyroxine for the Treatment of Hypothyroidism. Eur Endocrinol. 2013;9(1):40-47.
  2. Kansagra S, McCudden C, Willis M. The Challenges and Complexities of Thyroid Hormone Replacement. Laboratory Medicine. June 2010; 41(60): 338–348.
  3. What do you mean by the half life of a drug? Available at: [Accessed September 3, 2021].

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