I've been on thyroxine for hypothyroidism since it was diagnosed while I was TTC. I had low supply and a really big delay in my milk coming in for multiple reasons (induction, fast labour, attachment problems, PCOS and forgetting to reduce my thyroxine dose after the birth.) For me a combination of motilium, frequent feeds, pumping between feeds and lactation cookies did the trick. I was able to stop giving top-ups and DD is still a boobie girl 6 months on.

My advice would be to get your thyroid under control (consider seeing an endocrinologist if you don't think your GP is up to the task) and seriously consider trying motilium.