Placed on synthroid (thyroxine) and prednisone the same day for thyroiditis and insufficiency. i started collapsing when stressed. any explanations or thoughts?

Probably normal: Need to see your physician this could be a normal response from those two medicines, but need more follow up and evaluation.

See a Specialist: If you have 1 autoimmune syndrome (like hashimoto's thyroiditis) you are more likely to have another (like addison's disease). If you have addison's disease and you do not also take a mineralcorticoid (florinef), your blood pressure can become low, especially when stressed, which can prevent enough blood going to the brain and cause you to pass out. You need to alert your dr. Of your symptoms!