Does Levothyroxine Interact With Birth Control Pills?

Answer

While there isn't a drug interaction between levothyroxine (brand name Synthroid) and birth control pills directly, if your birth control pills contain estrogen, it can potentially have a significant impact on your blood thyroid levels.

Estrogen Containing Birth Control Pills - Levothyroxine Interaction

Levothyroxine is a prescription thyroid medication indicated for the treatment of hypothyroidism or low thyroid levels. It doesn't directly interact with birth control pills, but as mentioned above, estrogen-containing birth control pills can affect thyroid levels which could possibly necessitate an alteration on your levothyroxine dose.

Estrogen has been shown in studies to increase thyroxine-binding globulin (TBG) concentrations in the blood. TBG is a transport protein that binds thyroid hormones (both T₄ and T₃) in the blood. Bound thyroid hormone is not active and could lead to symptoms of hypothyroidism including:

• Chills
• Lethargy
• Hair loss
• Lack of energy

The estrogen from birth control pills can potentially cause an increase in TBG. This increase can lead to more thyroid hormone being bound to it, making less available to be utilized in the body. This can in turn, lead to stimulation of thyroid stimulation hormone (TSH), which activates when free thyroid concentration drops too low. This cascade of events can potentially alter the dosage needs of your prescription levothyroxine.

In summary, estrogen can cause the following:

• Increase in TBG

• Increase in bound thyroid hormone (to TBG)

• Decrease in free thyroid hormone

• Symptoms of hypothyroidism

• Stimulation of TSH

Separate Levothyroxine Administration From Other Drugs

Lastly, although there is no interaction between levothyroxine and birth control pills, they should not be administered at the same time to avoid decreases in the absorption of your levothyroxine dose. Levothyroxine should ideally be administered as follows:

• Take your dose of levothyroxine 30 minutes before food, drink or other medication. It should also be taken with at least 8 ounces (one cup) of water.

Having said all of the above, if you have been taking your birth control pills and levothyroxine together for some time, it may be prudent to stick with your current therapy and make no changes.

The most important aspect of levothyroxine dosing is to be consistent. Your thyroid levels may be normalized based on how you have been taking your levothyroxine and birth control pills. Before you make any changes to your current regimen, you should discuss it with your doctor.