
Are we talking about Soloxine? It's a well known fact (to those of us with an underactive thyroid like myself, anyhow!) that it is very difficult to start or maintain a pregnancy if you are hypothyroid, so the levels of T3 and T4 have to be within a certain range for pregnancy to occur at all. So it's logical to keep taking the tablets!
Talk with your vet, obviously. In humans, the blood levels are monitored regularly to assess the dosage; when we had a flatcoat with the same problem, they tended to assess dosage by her weight and response to treatment, which I felt was nothing like accurate enough. Having said that, she lived a healthy, happy life on Soloxine, and died at 12.5 years . The hypothyroidism was diagnosed when she was about six years old, and had already bred.
I believe there is a hereditary aspect to thyroid problems, and we had bred before it was diagnosed ( with both myself and the bitch!) so it's as well to bear this in mind with any progeny, and to forewarn any potential puppy people if she's in whelp already.
Jo and the Casblaidd Flatcoats