OverviewDosageSide EffectsInteractionsHalf-Life

Levothyroxine was the third most commonly prescribed drug in the United States in 2017. In that year alone, there were over 101,932,430 prescriptions. Considering how popular this drug is right now, it pays to know a little bit more about this medication, including how long levothyroxine stays in your system.

The U.S. Food and Drug Administration (FDA) states the following information regarding levothyroxine half-life:

Mean effective half-life: 9 to 10 days

Time to maximum concentration: 2 to 3 hours

Time to steady state: 6 weeks

Accumulation ratio: N/A (drug does not accumulate in the body)

Levothyroxine has an oral bioavailability of up to 80 percent, and the body more readily absorbs the drug when taken on an empty stomach. The bioavailability may be lower in elderly patients. Levothyroxine is also available as an injection, but its bioavailability is less clear (estimated by the FDA to be closer to 48 to 74 percent).

When taken orally in pill form, most of levothyroxine is absorbed by the small intestine. The drug is metabolized by the liver and kidneys and primarily excreted via the kidneys through urine, although some is also excreted in feces.


What is the half-life of levothyroxine?

The half-life of levothyroxine is approximately 9 to 10 days in people with hypothyroidism.

How long does levothyroxine stay in your system?

Levothyroxine is detectable in your system for about 4 to 6 weeks. Your medical provider may order periodic blood tests to detect the level of levothyroxine in your system in order to monitor for adverse reactions or make dosage adjustments, especially if you have other health conditions or take any other medications.

How long does it take for thyroid medication to wear off?

Once a person has started taking levothyroxine, it can take about 4 to 6 weeks for a person with hypothyroidism to begin noticing an improvement in their symptoms. Unless their hypothyroidism is caused by a temporary issue, most people end up taking thyroid replacement hormones for life. This fact is because levothyroxine can treat but not cure hypothyroidism. If they stop taking thyroid medication, it can take up to 6 weeks for levels in their body to drop enough to have no significant effect. 

How long does it take for your body to adjust to levothyroxine?

Levothyroxine starts working right away and peaks in your system within about 2 to 3 hours after taking a prescribed dose. However, it can take about 4 to 6 weeks for the body to adjust to the medication and for you to start seeing improvements in your hypothyroid symptoms.

Disclaimer: this article does not constitute or replace medical advice. If you have an emergency or a serious medical question, please contact a medical professional or call 911 immediately. To see our full medical disclaimer, visit our Terms of Use page.

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