Levels of parathyroid hormone (PTH) should be considered when evaluating the effect of vitamin D status on glucose metabolism, according to research published online in Diabetes.

Caroline K. Kramer, M.D., Ph.D., of the University of Toronto, and colleagues performed serial metabolic characterizations in 494 women during the postpartum period to evaluate the associations of vitamin D and PTH levels with glucose metabolism.

The researchers found that 32 percent of the women with prediabetes or diabetes at 12-months postpartum had both vitamin D deficiency and elevated PTH (in the highest tertile) at three-months postpartum. Using multiple-adjusted linear regression models, vitamin D insufficiency/deficiency combined with PTH in the highest tertile at three months was an independent predictor of poorer beta-cell function (P = 0.03) and insulin sensitivity (P = 0.01), and increased fasting (P = 0.03) and two-hour glucose (P = 0.002) at 12-months postpartum. Vitamin D insufficiency/deficiency with lower levels of PTH did not predict these outcomes.

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“In conclusion, only vitamin D deficiency/insufficiency with increased PTH is an independent predictor of beta-cell dysfunction, insulin resistance and glycemia, highlighting the need for consideration of the PTH/25-hydroxyvitamin D axis when studying the impact of vitamin D status on glucose homeostasis,” the authors write.