Seasonal Variability in Vitamin D Levels No Longer Detectable in Primary Hyperparathyroidism

Seasonal variability in 25-hydroxyvitamin D [25(OH)D] and PTH levels in the general population has been associated with differences in bone turnover markers, bone density, and fracture risk. Seasonal variability in 25(OH)D and PTH levels has also been reported in primary hyperparathyroidism (PHPT).

Given the widespread use of vitamin D supplements, we sought to determine whether patients with PHPT still demonstrated seasonal variation in 25(OH)D levels.

This cross-sectional study was conducted at a university medical center at a Northeastern U.S. latitude (New York, NY).

One hundred patients with PHPT participated in the study.

We assessed vitamin D supplement use and seasonal variation in serum 25(OH)D.

Patients had PHPT ([mean ± SD] calcium, 10.8 ± 1.0 mg/dL; PTH, 85 ± 48 pg/mL) with a mean 25(OH)D level of 29 ± 10 ng/mL. Although only one fifth of participants had vitamin D deficiency (19% < 20 ng/mL), more than half were either deficient or insufficient (54% < 30 ng/mL). Sun exposure varied by season, but there were no seasonal differences in levels of 25(OH)D, PTH, bone markers, or bone mineral density, or in the prevalence of 25(OH)D less than 20 or less than 30 ng/mL. Most of the participants (65%) took supplemental vitamin D (dose among users: mean, 1643 ± 1496 IU; median, 1000 IU daily), and supplement users had markedly better vitamin D status than nonusers (25(OH)D < 20 ng/mL: 8 vs 40%; P < .0001; < 30 ng/mL: 40 vs 80%; P = .0001; ≥ 30 ng/mL: 60 vs 20%; P = .0001).

We found no evidence of seasonal variation in 25(OH)D levels or PHPT disease severity in the Northeastern United States. This change is likely due to widespread high vitamin D supplement intake, which has resulted in better vitamin D status among supplement users and can mask the effect of season on serum 25(OH)D levels.