What Does Levothyroxine Do? All About Its Mechanism of Action
Key takeaways:
Levothyroxine provides your body with a man-made version of thyroxine (T4), a type of thyroid hormone. T4 gets converted in the body to triiodothyronine (T3). This is the “active” thyroid hormone that helps your body function normally.
When levothyroxine starts working, symptoms of hypothyroidism should improve. These include feeling cold, tired, and low on energy.
If you’re taking levothyroxine and start to have symptoms of hypothyroidism or hyperthyroidism, your levothyroxine dosage may need to be adjusted. Let your healthcare provider know if you experience any symptoms.
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Levothyroxine (Synthroid, Levoxyl, Tirosint) is the go-to treatment for hypothyroidism. This is a condition where your body doesn’t produce enough thyroid hormones. Thyroid hormones are necessary to help your body function normally. They’re involved with processes like maintaining body temperature and keeping your energy levels up.
Levothyroxine is one of the most commonly prescribed medications in the U.S. If you’re one of the millions of people with hypothyroidism, taking levothyroxine may be able to help you feel normal again.
But exactly what does levothyroxine do? Let’s take a closer look at levothyroxine’s mechanism of action and how it helps balance thyroid hormones.
What does levothyroxine do?
Levothyroxine works by providing your body with a man-made version of thyroxine (T4), a type of thyroid hormone.
This is helpful because hypothyroidism is a medical condition where the body doesn’t make enough thyroid hormones. If you or your healthcare provider suspects you have hypothyroidism, they’ll order a blood test to check your levels of a few hormones, including:
Thyroid stimulating hormone (TSH): TSH is produced by the pituitary gland, which is located at the base of the brain. TSH sends a message to the thyroid gland to produce thyroid hormones. When you have enough thyroid hormones, the pituitary gland decreases how much TSH it produces. TSH can be low or high in hypothyroidism, but it’s usually high.
T4: This is a thyroid hormone made by the thyroid gland. In hypothyroidism, T4 levels are usually low. Levothyroxine is a man-made version of T4.
Triiodothyronine (T3): This is another thyroid hormone made by the thyroid gland. Most T3 in the body comes from the conversion of T4 to T3. T3 is the “active” form of thyroid hormone that your body can use. In hypothyroidism, T3 is usually low.
When you take levothyroxine orally, it provides your body with T4. Your body then converts the T4 to T3.
Does levothyroxine start working immediately?
Not necessarily. It can take 4 to 6 weeks to feel the full effects of levothyroxine. But some people may notice a difference in their hypothyroidism symptoms sooner — within just a few days.
How do you know if levothyroxine is working?
When levothyroxine starts working, you’ll feel symptoms of hypothyroidism improve. For example, it’s common to feel tired or have dry skin if you have untreated hypothyroidism. After starting levothyroxine, you should start to have more energy, feel more alert, and see less dry skin. Other symptoms that may improve after starting levothyroxine include:
Feeling cold when others don’t
Constipation
Depression
Weight gain
Hair loss
Sexual problems
Muscle weakness and cramps
How do you know if your levothyroxine dosage needs to be adjusted?
There are two main ways to know if your levothyroxine dosage needs an adjustment:
You have symptoms of hypothyroidism or hyperthyroidism.
Your lab results show your thyroid hormone levels aren’t within your target range.
If you start feeling symptoms of hypothyroidism discussed above, this might mean your levothyroxine dosage is too low. On the other hand, if you start feeling symptoms of hyperthyroidism, this might mean your levothyroxine dosage is too high.
Symptoms of hyperthyroidism include:
Nervousness
Weight loss
Irritability
Trouble sleeping
Fast heart rate
Feeling weak
Brittle (thinning) skin
If you start to experience any symptoms of hypothyroidism or hyperthyroidism, let your healthcare provider know. They’ll likely order a blood test to check your TSH levels.
If you take levothyroxine, your provider will likely recommend checking TSH levels one to two times a year (even if you feel great). If your TSH is high, you may need a higher dose of levothyroxine. If your TSH is low, you may need a lower dose of levothyroxine.
Don’t adjust your levothyroxine dose without talking to your healthcare provider. If they make any dosage adjustment, they’ll likely recheck your TSH levels in about 2 months.
Good to know: Some situations may require closer monitoring of your levothyroxine dosage. For example, if you have hypothyroidism and become pregnant, your TSH levels will be checked more frequently. In fact, most people with existing hypothyroidism will need a higher dose of levothyroxine during pregnancy.
The bottom line
Levothyroxine is the go-to treatment for hypothyroidism. It’s a man-made version of thyroxine (T4), a type of thyroid hormone. When you take levothyroxine, it enters your body and becomes converted to triiodothyronine (T3). T3 is considered the “active” thyroid hormone — the one that helps your body function normally.
Levothyroxine can take a few weeks to work. But you may feel your hypothyroidism symptoms start to improve within just a few days. This could mean feeling more energetic and not feeling cold as frequently. But if any of these symptoms start to return, or you feel symptoms of hyperthyroidism (like nervousness and brittle skin), your levothyroxine dosage may need to be adjusted.
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References
A-S Medication Solutions. (2023). Levothyroxine sodium tablet [package insert].
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