I'm due to have a thyroid scan in 3 days. Yesterday, I took two 25 mg antihistamine pills. Will this affect the results?

No, it should not. The half life of most antihistamines should be less than 24 hrs and should be out of the system by then. I would check with the radiology center to confirm their policy. I would also avoid any more antihistamines until the time of testing.
No. There are a couple of different agents utilized for thyroid scans, radioiodine and tc99m pertechnetate. A couple of antihistamine pills should not have any effect at all upon your thyroid scan.
No. As long as the medicine or even the food you take prior to the study does not have iodine as one of the ingredient, it does not affect the results.
No it should be fine. Antihistamines do not affect thyroid scans, any radioactive scans or contrast studies.