Synthroid and Birth Control: Key Interactions
Birth control pills do not interfere with Synthroid (levothyroxine) effectiveness, but birth control may increase your levothyroxine requirements by raising thyroid-binding globulin levels, necessitating TSH monitoring and potential dose adjustment. 1, 2
The Primary Interaction: Birth Control Affects Levothyroxine Needs
Estrogen-containing contraceptives increase thyroid-binding globulin (TBG) production in the liver, which binds more thyroid hormone and reduces the amount of free, active hormone available to your tissues 1, 2
This interaction means women starting birth control while on levothyroxine may need a higher dose of their thyroid medication to maintain the same therapeutic effect 1, 2
The FDA label for levothyroxine explicitly lists "estrogen-containing oral contraceptives" as drugs that may increase serum TBG concentration, resulting in an initial transient increase in free T4, followed by a decrease in serum T4 with continued administration 1
Clinical Management Algorithm
When starting birth control on stable levothyroxine:
- Check TSH levels 6-8 weeks after initiating oral contraceptives 1
- Increase levothyroxine dose if TSH rises above target range 1
- Recheck TSH 6-8 weeks after any dose adjustment 1
When stopping birth control on stable levothyroxine:
- Monitor TSH 6-8 weeks after discontinuation 1
- Decrease levothyroxine dose if TSH becomes suppressed, as TBG levels will fall 1, 2
Important Distinction: Levothyroxine Does NOT Reduce Contraceptive Efficacy
Levothyroxine is NOT listed among drugs that reduce contraceptive effectiveness in the 2024 CDC contraceptive guidelines 3
The interaction is unidirectional: birth control affects thyroid hormone binding, but thyroid medication does not induce enzymes that metabolize contraceptive hormones 1, 2
This contrasts sharply with rifampin, which is a potent enzyme inducer that dramatically reduces contraceptive efficacy and requires backup contraception 3, 4
Critical Pitfall to Avoid
Do not confuse levothyroxine with rifampin or other enzyme-inducing drugs. While rifampin appears in tuberculosis treatment guidelines alongside levothyroxine in drug interaction tables, rifampin reduces contraceptive effectiveness (requiring barrier methods), whereas levothyroxine does not affect contraceptive efficacy at all 3, 4
Progestin-Only Methods
Progestin-only contraceptives (pills, implants, IUDs) have minimal effect on TBG compared to estrogen-containing methods, so the interaction with levothyroxine is less pronounced 2
Women on levothyroxine who prefer to avoid potential dose adjustments may consider progestin-only methods, though TSH monitoring remains appropriate 2
Additional Cardiovascular Consideration
Women with subclinical hypothyroidism or those on levothyroxine may have increased cardiovascular and thromboembolic risk when taking estrogen-containing contraceptives, as hypothyroidism is associated with higher mean platelet volume and platelet hyperactivity 2
Ensure hypothyroidism is adequately treated (TSH at goal) before initiating or continuing estrogen-containing contraceptives to minimize this risk 2