It depends - What effect is it having on you?
are you depressed, or are you always tired and falling asleep? And are you undergoing treatment?
If you are currently depressed then I suspect that would bar you from entry until you get the all clear from your doctor.
But there are obvioulsy a lot of factors and points to take on with an underactive thyriod.
The best advice I can give you is go talk to the unit you want to join - Perhaps with some written documentation from your doctor with specific details and go from there.
Theres no harm in asking, and getting down to the unit and having a chat is ultimatley the only way you will find out for sure.