What is the T score for severe osteoporosis?

Could you elaborate on the significance of the T score in assessing the severity of osteoporosis? Specifically, I'm curious to know what range of T score values indicate severe osteoporosis. Is there a specific threshold that medical professionals use to classify it as severe? Additionally, how does the T score correlate with bone density and fracture risk in patients with osteoporosis? Is it a reliable indicator in clinical practice? I'm also interested in understanding the potential impact of severe osteoporosis on patients' quality of life and treatment options available.