Readings or data have anomalies; organs function abnormally would be my take.
So if a study was looking at large numbers of tests of thyroids, it would likely be looking for anomalies. On the other hand, if a doctor or medical examiner was examining a single thyroid, he would be be seeking to determine if it is in any way abnormal.
I think abnormality works as both a count or a non count noun. "No abnormality was found" and "no abnormalities were found" are both correct. The first has the virtue of brevity, so use that.
But I'm just a writer with a good ear. If this question matters professionally to you, you might want to check what other medical documents say before you take my word for it. Actual usage in the field might differ from my instincts. I am not a doctor. If you are doing this professionally, you want to conform to them, not to me. Even if I'm right.