Background Primary hyperparathyroidism (PHPT) and vitamin D insufficiency are common conditions that can occur in combination. However, low plasma 25-hydroxyvitamin D (25OHD) may also enhance the risk of PHPT or modify disease severity.
Aim To compare the risk of vitamin D insufficiency and deficiency stratified by age, sex and season between PHPT patients and controls and to assess associations between plasma 25OHD and adenoma weight, biochemical variables, bone mineral density (BMD) and clinical complications.
Design Cross-sectional study.
Material A total of 289 consecutive Caucasian patients with PHPT aged 65·9 (24–92) years, 289 sex-, age- and season-matched normocalcaemic controls and 187 healthy adult blood donors. PHPT diagnosis was confirmed in 214 by neck exploration.
Results Vitamin D insufficiency (plasma 25OHD < 50 nmol/l) was observed in 81% of PHPT patients compared with 60% of sex- and age-matched controls (P < 0·001) and 35% of blood donors (P < 0·001). During summer, 77%vs. 53% (P < 0·001) and 4% (P < 0·001), respectively, had vitamin D insufficiency. Average plasma 25OHD was 41 (range 9–87) nmol/l among 27 PHPT patients compared with 87 (21–173) nmol/l (P < 0·001) among aged-matched blood donors. During winter, 86%vs. 66% (P < 0·001) and 71% (P < 0·05), respectively, had vitamin D insufficiency. Vitamin D deficiency (plasma 25OHD < 25 nmol/l) was observed in 33% of PHPT patients compared with 20% of age- and sex-matched controls (P < 0·001) and 13% of blood donors (P < 0·001). Both PHPT patients and controls showed seasonal variations in 25OHD related to the average number of sun hours, but values were lower in PHPT patients at all calendar months. In PHPT patients low plasma 25OHD was associated with higher plasma levels of calcium, PTH and alkaline phosphatase and with lower renal calcium excretion, femoral neck and forearm BMD. No association was found between plasma 25OHD and adenoma weight (total or divided into tertiles). There was a trend towards increased risk of osteoporotic fractures (P < 0·08) with low plasma 25OHD.
Conclusion Vitamin D insufficiency and deficiency are common findings in PHPT and occur more often than in a sex- and age-matched control group referred from general practice and in normal blood donors irrespective of season. Low plasma 25OHD levels are associated with an aggravated clinical presentation of PHPT but do not affect adenoma size.