Could there be a connection between oral thrush and hypothyroidism?

Not usually: There is a rare condition called autoimmune polyendocrine syndrome, type 1 in which both hypothyroidism and a mild immunodeficiency (susceptibility to yeast infections, including oral thrush) can occur together. One episode of thrush would not raise suspicion for this necessarily, but if you have had multiple episodes, you should discuss this with your doctor.