Hypothesis
Correcting vitamin D deficiency with daily supplements would lower the rate at which Graves' disease comes back after anti-thyroid drugs are stopped.
What failed
Of 210 vitamin D-deficient patients followed for at least a year after stopping anti-thyroid drugs, the 60 who took supplements relapsed at a rate not significantly different from the 150 who did not (38% vs 49%, P = 0.086), although relapse came somewhat later in the supplemented group.
Broken assumption
Raising vitamin D levels is enough to measurably prevent Graves' relapse.
Why
The authors read the difference as a borderline protective trend; the groups were self-selected rather than randomised, and vitamin D level at drug withdrawal still tracked with relapse in multivariate analysis.
From the published paper: Cho et al. Scientific Reports, 2020. ↗CC BY