Seasonal changes in serum calcium, PTH and vitamin D levels in patients with primary hyperparathyroidism

Background: Seasonal variations of 25-hydroxyvitamin D, PTH and calcium levels are not well characterized in primary hyperparathyroidism (PHPT). Our objectives were to characterize seasonal changes in these parameters in PHPT patients, and to assess whether these seasonal changes affect clinical decision making. Methods: This is a retrospective study based on the electronic medical records of Clalit Health service in the south of Israel between 2000 and 2012. Patients 18 years and older with PHPT (PTH > upper limit of norm (ULN) and serum calcium > 10.5 mg%) were included. Patients with renal failure or on Thiazide diuretics were excluded. All serum levels of calcium, PTH and 25-hydroxyvitamin D were collected and then stratified according to season. Results: 792 patients were classified as PHPT (72.2% female) and had a total of 2659 PTH tests, 1395 25-hydroxyvitamin D tests and 7426 calcium test. Fifty six percent of 25-hydroxyvitamin D levels were <. 50 nmol/L. Seasonality was demonstrated in all three parameters: mean 25-hydroxyvitamin D was 13% higher in the summer compared to the winter (P < 0.001), median PTH values showed opposite trend with a fall of about 8.4% in summer compared to winter (P < 0.001). Calcium levels were higher during the autumn with a rise of about 0.2 mg/dL in the mean calcium levels compared to spring and summer (P < 0.001). The odds ratio of calcium level above 11.5 mg/dL is highest in the autumn (OR = 1.275, P = 0.018). Conclusion: We show seasonal variation in serum 25-hydroxyvitamin D, PTH, and calcium levels in patients with PHPT. These seasonal variations cause transition to pathological values that may influence diagnosis and treatment of PHPT patients.