Mary1962, I can only give you my personal experience on this. I have never had an underactive thyroid but can tell you that my thyroid function is better when I take more oestrogen. When I didn't take HRT at all and when my oestrogen levels were very low while taking HRT, my thyroid levels were getting quite near to the upper limit but since I switched to the gel and my oestrogen blood level was higher, my thyroid function improved - I always have the whole set of thyroid blood tests. This has happened too many times for it to be a coincidence in my case. That said, it might well be different for some people.