Long term use of Fentanyl causing hormonal problems?

I have suffered chronic back pain for 4.5 years now and having tried a wide range of pain relief have been using the fentanyl patches 100mcg for approx 18 months. I had an appointment at my pain clinic and the consultant started telling me I was having this problem and that problem, from muscle troubles to thyroid problems. She then sent a letter to my GP stating that I was having hormone trouble because of the use of the Fentanyl and she wants to reduce it. I have had extensive blood tests and the only problem found was raised cholesterol. My GP did not understand the reference to hormonal problems due to the use of Fentanyl. In spite of the consultants suggestions of these problems, other than chronic pain and depression I don't think I have any of the problems she's suggesting. Has anyone heard of the long term use of Fentanyl causing hormonal problems?