Hyperthyroidism

I had a cat diagnosed as hyperthyroid about 3 months ago. Since then, I have had 3 more diagnosed with hyperthyroid. I know it is not contagious but is that unusual? I have another one with the same symptoms that I am getting lab work done on this week. I just think it is strange that all my cats are becoming hyperthyroid. This would be the 5th one in as many months. Anyone else have this happen?

That sounds very strange. Are they related? How old are they? This article discusses some of the possible causes:

Why Has Hyperthyroidism in Cats Reached Epidemic Levels?

They are not related. They are Midget SF 14yrs, Preston NM 11yrs, Torrence SF 16yrs, Rat NM 16yrs, and Fred NM 13yrs (waiting on labs for Fred). All of my cats are seniors. I feed Purina Indoor Dry and Friskies canned food, Temptations and Greenies treats. I use clumping clay litter. All cats are strictly indoors, but I do put Frontline Flea and tick on 2x a year, spring and fall, due to dog. I did read the article with your post, interesting, but difficult to know which is the culprit and difficult to experiment with many cats.