whats thyroid deficiency?

i think my dog may have it. I posted about the dog shampoo. She shows some symptoms: Hair loss, Hair condition changes, dry skin (dandruff), diarrhea, some climatic sensitivity (she’s always had that though, she’s a Boston :), but I think it’s been getting a little worse, but I just thought that it was because of the hair lose. I don’t know. Do you think she could have it, or is there something that they always have that I didn’t mention.

I’m taking her to the vet this week anyways, and I’ll bring this up, but can someone tell me what the deal is with thyroid deficiency is.

Also: some websites said that once you put your dog on it you can’t take it off. Why is that?

Also: My dog is allergic to protein, or something in it, so she has to have a special Science Diet food. She’s been on that for a loong time. Without it, she barfs everywhere.

ahhhh
no wieght gain at all. she’s otherwise extremely healthy
yes, i’m talking about hypothyroidism
and she has bad cataracts (spelling?) could that be related?