Effect of testosterone replacement therapy on vitamin D and FGF-23 levels in congenital hypogonadism

Introduction: Patients with hypogonadism are at increased risk of cardiac and metabolic diseases and osteoporosis. Vitamin D and Fibroblast growth factor-23 (FGF-23) play role in the regulation of bone mineral metabolism and endothelial functions. Low vitamin D levels are reported in hypogonadism, while there is no data about the effect of testosterone replacement therapy (TRT). We investigated the effect of TRT on vitamin D and FGF-23 levels along with endothelial functions and insulin resistance in hypogonadal patients. Material and methods: Patients with congenital hypogonadotrophic hypogonadism (CHH) (n=32, age 20.6 ±1.58 years) were enrolled. TRT was implemented in transdermal form. The demographic parameters, FGF-23, 25(OH)D3, Asymmetric dimethylarginine (ADMA) and homeostatic model assessment of insulin resistance (HOMA-IR) levels were measured both before and after TRT. Results: After a follow-up period of 3.63±1.33 months, ADMA and FGF-23 levels were significantly increased (p=0.03 and p=0.005 respectively), while the 25(OH)D3 and HOMA-IR index were not significantly changed. The body mass index and waist circumference levels of the patients were also increased (p<0.001 and p=0.02) along with a significant decrease in the HDL cholesterol levels (p=0.006). Conclusions: The results show that a short term TRT increases plasma FGF-23 and ADMA levels, in young, treatment naive patients with CHH. Whether this is an early implication of TRT related adverse effects in this very young and treatment naive population of CHH is not clear. Future prospective studies are required to find out the long-term effects of TRT on cardio-metabolic morbidity and mortality in this specific population.