Hi, Gilbertine!
It is known that introducing HRT can throw the thyroid function off balance, and that your thyroid medication may need to be adjusted.
I am also a member of Hystersisters (all the ladies there have had hysterectomies and or oophorectomies though), but the same applies. I have read many posts there about ladies who are balancing thyroid hormone and HRT, and in most cases an adjustment in HRT or addition of HRT resulted in a requirement to adjust their thyroid medication.
Rest assured that you can indeed take HRT though! I think your GP needs to be a little thoughtful about this, and if you are currently seeing a Consultant Endocrinologist then his input is a must. If not, then your GP should probably make a referral for a revue of your situation. Maybe phone your GP next week and arrange to discuss this, armed with this info. I am sure there are MM members too who are balancing thyroid medications and HRT. Hopefully one of them will be along shortly to add their thoughts, or maybe start a new thread with Thyroid/HRT as a title so that it draws theiir attention.
Hope this helps, and good luck!