Synthroid with the birth control pill
Many women of reproductive age take a combined or progestogen-only oral contraceptive while also using a chronic medication such as Synthroid (Levothyroxine). The combination is generally fine at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg, but a small number of medications can reduce contraceptive efficacy meaningfully and need either a backup method or a switch.
How Synthroid can affect contraceptive efficacy
Combined and progestogen-only contraceptives are metabolised through CYP3A4. Strong CYP3A4 inducers (some antiepileptics, rifampicin, St John's Wort) lower contraceptive plasma levels and reduce efficacy. Whether Levothyroxine acts on CYP3A4 determines whether Synthroid affects contraception. Most agents in Hormones and Birth Control have no clinically meaningful effect on the pill at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg.
Practical guidance
According to the prescribing information for Levothyroxine, women on hormonal contraception should review Synthroid with the prescribing pharmacist or doctor. Where an interaction is documented, additional barrier contraception or switching to a non-oral method (IUD, implant) for the duration of Synthroid therapy is the standard mitigation.
Frequently asked questions
Will Synthroid make my pill less effective? ▾
Most Hormones and Birth Control medications at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg do not affect oral contraceptive efficacy. The exceptions are CYP3A4-inducing drugs and a small number of others. The prescribing information for Levothyroxine states whether the interaction is meaningful.
Do I need a backup contraceptive on Synthroid? ▾
Backup contraception is needed only when there is a documented interaction between Synthroid and the contraceptive method. For most users at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg, no backup is required. The pharmacist confirms whether Levothyroxine interacts with hormonal contraception.
More on Synthroid
- With alcoholSynthroid and alcohol — is it safe to drink?
- With foodShould Synthroid be taken with food?
- Side effectsSynthroid side effects: common, rare and warning signs
- Dosage guideSynthroid dosage guide: how much to take and when
- OnsetHow fast does Synthroid start working?
- DurationHow long does Synthroid last?
The information on this website is provided for reference and educational purposes only. It does not replace consultation with a qualified healthcare professional.