Can a Veterinarian refuse to treat an animal with a life threatening condition.?

I have been taking my pets to the same veterinary in Gananoque,Ont. for the last 25 years, this past Tuesday June 7th I called to get a blood test for my 8 year old dog who has a Thyroid condition which requires her to take medication twice a day, this blood test is vital to determine if the medication that she is taking needs to be increased or decreased. They informed me that she hasn't had a full exam for 3 years ( eyes ears nose etc. ), when I told them that I could only afford the blood test plus her medication which she needed a refill for because she only had enough for 5 more days plus some flea and tick medication I was told no exam no nothing, ( including the flea and tick medication which you don't even need a veterinarian for. ). Besides her thyroid condition she is healthy and has always been taken in for an exam when anything is wrong with her. Is refusing to treat my dog because I can't afford the exam on top of everything else that I HAVE to buy which is expensive ( over $300 ) ethical or even legal. Luckily there is another Veterinary clinic in town which opened a few years ago who had no problem with giving her a blood test as well as her medication without an exam the very next day.