While this is reported in the handout on
HCTZ, there is not much published in the clinical thyroid literature about an effect on thyroid levels and I have not noted in practice that HCTZ alters thyroid levels in any significant way. So it is unclear if it had an effect in your case. An US could be done to see if there is any significant thyroid tissue -- that would not prove it is active tissue, but be a starting point to see how much is there.