Can birth control pills affect thyroid function test results?

Yes, birth control pills containing estrogen can affect thyroid function test results. Estrogen increases the levels of thyroid-binding globulin (TBG), a protein that binds to thyroid hormones, making them less available for the body to use. This can cause T4 levels to appear higher than they actually are, even though free T4 (the active form) remains normal. If you’re on birth control, it's important to inform your doctor, as they may interpret your thyroid test results accordingly. In some cases, additional tests may be needed to accurately assess thyroid function while using hormonal contraceptives.