Synthroid and grapefruit: a real interaction?
Grapefruit is famous as the juice that interacts with medications, and the warning is real for a number of drugs. Whether it matters specifically for Synthroid (Levothyroxine) at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg depends on whether the active ingredient Levothyroxine is metabolised by CYP3A4 in the gut wall and how much that pathway contributes to first-pass metabolism.
The CYP3A4 mechanism
Grapefruit (and its juice) inhibits CYP3A4 enzymes in the gut wall, increasing the absorbed dose of medications metabolised by that enzyme. For drugs in the Hormones and Birth Control class that go through CYP3A4 first-pass metabolism, regular grapefruit consumption can raise plasma levels of Levothyroxine by a clinically meaningful margin and amplify side effects.
What to do with Synthroid
According to the prescribing information for Levothyroxine, if grapefruit is flagged as an interaction, the safest practice is to avoid grapefruit entirely or to discuss timing with the prescriber. Levothyroxine replaces deficient endogenous thyroxine, which is converted in tissues to the active hormone triiodothyronine (T3) by deiodinase enzymes. A small occasional serving may be tolerable; daily large servings around the time of Synthroid dosing are not recommended at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg.
Frequently asked questions
Can I eat grapefruit while on Synthroid? ▾
For some Hormones and Birth Control medications, grapefruit is best avoided or limited because of CYP3A4 inhibition. Whether Synthroid specifically is affected depends on Levothyroxine; the prescribing information lists this. When in doubt, ask the pharmacist or check the patient leaflet.
How much grapefruit is too much with Synthroid? ▾
For drugs where the interaction matters, even modest daily grapefruit intake (one whole grapefruit or one large glass of juice) can shift drug levels noticeably. For Synthroid at 25mcg, 50mcg, 75mcg, 100mcg, 150mcg, individual tolerance varies; the cautious choice is to avoid grapefruit if any interaction is mentioned.
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.