I was told cysts do not always show up on ultrasound. That they can often be mistaken as eggs, and eggs can be mistaken as cysts. Go to your OBGYN, ask for the hormone testing associated with PCOS. I believe it is 3 different tests, one is testosterone, the other is FSH and the other is LH. They have to be taken on the 2nd or 3rd day of your period for the correct results. Based on that and the ultrasound and your symptoms they should be able to tell you for sure if you have PCOS.
On the other hand, I'm not sure what you mean by taking tablets. Medication for the under active thyroid I'm assuming...
Cyst should show up with a ultrasound. Just tell gynecologist or doctor your symptoms and see if they advise one. That is how I found out about mine. Good luck.