1,25-dihydroxy-vitamin D3 (1,25 (OH) 2D3) or calcitriol regulates plasma calcium and phosphate concentrations in the blood by stimulating intestinal absorption and renal reabsorption of calcium and phosphate. 1.25 (OH) 2D3 is formed by the renal 1α25OH vitamin D-hydroxylase (Cyp27b1) and inactivated by the renal 25-hydroxy vitamin D-24-hydroxylase (Cyp24). Unpublished studies in mice revealed an inhibitory effect of 1.25 (OH) 2D3 on embryo implanation. An excess of 1.25 (OH) 2D3 could therefore lead to female infertility. The excess of 1.25 (OH) 2D3 could result from uncritical vitamin D intake with insufficient negative feedback by FGF23 / Klotho. The aim of the study is to identify patients with an excess of 1.25 (OH) 2D3.