Was the thyroid panel sent to an external laboratory? Depends on which form of Thyroid condition is present? Lymphacitic Thyroiditis is hereditary. Yes, its treatable with meds with great response on the dog, but don't count on getting a girl in whelp with pups. If you do, count on having issues keeping girl in whelp and having a normal healthy litter.
Personally, I don't breed a dog past 6, so thyroid issue or not, she is out of my program at that age. Thyroid issues are common though...I had an older bitch go on meds around 7 and an older male around 8.
I would speak to Jean Dodds. How low was "low"? Did you do a full panel testing or just a T4? As far as age, I do last breedings at 6 if the bitch has produced well in the past. I know of alot of breeders that do okay at ages 7 & 8. I'm not comfortable breeding at those ages. My girls retire to any room they want to wander into at that age except formal rooms in the home. If super well behaved they always did from 2 on.
How did her prior offspring turn out? I think that's the important question. I don't think it's odd for a 6 year old's thyroid to lower at that age but if I test, I do it younger. If at age 2 and it were low, I wouldn't breed the girl ever and that only happened to me once in 20 years. She had dry coat & skin and was gaining weight for no reason.
Did she or any of your girls happen to seize at age 6 or at any time that might have been caused by low thyroid levels? If there were no symptoms what makes you test 6 year olds instead of younger? I'm just curious about that.
Only you can decide whether or not to breed her if your vet gave you the okay. Jean Dodds might be able to help you decide what to do.
