Context: Turner syndrome (TS) is characterized by short stature and premature ovarian insufficiency (POI). The main long-term complication of POI is osteoporosis, which can be prevented by hormone replacement therapy (HRT).

Objective: The objective of our study was to compare initial bone mineral density (BMD) and progression between TS and idiopathic POI patients under HRT.

Methods: A single-center retrospective study was conducted between 1998 and 2018. All women had undergone at least two bone densitometry assessments at least 2 years apart.

Results: 68 TS patients and 67 idiopathic POI patients were included. Mean age at initial assessment was 27 years (IQR, 21-35.5 years) in TS patients and 31.5 years (IQR, 23-37 years) in idiopathic POI patients (p=0.1). Lumbar and femoral neck BMD were lower in the TS group than in the idiopathic POI group (respectively 0.89g/cm² versus 0.95g/cm², p=0.03; 0.70g/cm² versus 0.77g/cm², p<0.0001). Mosaic karyotype was associated with better BMD in TS patients while history of growth hormone treatment had no impact on BMD. Over time, a significant gain in vertebral BMD was observed in TS patients versus a loss of BMD in idiopathic POI patients (p=0.0009).

Conclusion: TS patients had a lower BMD at baseline than idiopathic POI patients, at both spinal and femoral levels. Over time, on HRT, a significant gain in vertebral BMD was observed in patients with TS, compared with a loss of BMD in patients with idiopathic POI. We hypothesized that earlier initiation and longer duration of HRT played an important role in this finding. Long-term prospective follow-up to assess the incidence of fractures in TS would be useful.