Vitamin D and Birth Control
Estrogen-containing contraceptives raise serum 25(OH)D levels — not by increasing vitamin D synthesis, but by increasing vitamin D binding protein (DBP). Women switching off the pill often see 25(OH)D fall by 20% or more without any change in intake or sun exposure. Understanding this artefact matters for testing and pre-conception planning.
The Harmon 2016 finding
A cohort study by Harmon et al. (published in J Clin Endocrinol Metab) found women on combined oral contraceptives had mean 25(OH)D ~20% higher than women not on hormonal contraception — after adjusting for season, race, BMI, and vitamin D intake. Levels dropped predictably when women discontinued the pill in preparation for pregnancy.
Mechanism — DBP, not synthesis
Estrogen upregulates hepatic synthesis of vitamin D binding protein. Total serum 25(OH)D (which is what standard assays measure) is DBP-bound plus albumin-bound plus free. Raising DBP raises the measured total without necessarily raising the biologically active free fraction. This is one reason why 25(OH)D is not a perfect measure of vitamin D adequacy across all contexts.
Practical implications
- Pre-conception planning — women stopping the pill to conceive may see 25(OH)D drop 15–25% within a few months. Consider testing 3 months after discontinuation and correcting deficiency before pregnancy.
- Interpreting borderline levels — a woman on combined pills with 25(OH)D of 32 ng/mL may effectively have less functional vitamin D than her total suggests.
- Progestin-only methods (IUD, mini-pill, implant, injectable) do not appreciably affect DBP or 25(OH)D.
- Postmenopausal hormone therapy — estrogen-containing HRT has a similar (smaller) DBP effect.
Other hormonal considerations
- Pregnancy itself markedly increases DBP and 25(OH)D through the same estrogen pathway. Standards for interpreting 25(OH)D in pregnancy should account for this.
- Testosterone therapy has minimal effect on DBP.
- Corticosteroid use decreases DBP and often causes measured 25(OH)D to fall.
Should this change your birth-control decision?
No. The DBP effect is a testing artefact, not a reason to prefer one contraceptive over another. Contraceptive choice should be based on efficacy, side effects, medical history, and personal preference — not on vitamin D binding protein.