obesity

Question
My Golden ,Max,  is 4 yrs old. I received him when he was 3. He was under weight, and slobbering a lot. Within a week he stopped the slobbering and finally ate at a slow rate. Within 2mths, I had him neutered. After 2 weeks, he started to gain weight continously. He has been on a weight loss program {followed by his 2 vets.he is weighted every wednesday. He only eats twice a day for a total of 425 grams a day. But he is still gaining. I walk him for 20-30 min a day and have recently got a 4mth Red Fox Labrador. Thought this might hep but not much success. His health is deteriorating, problems walking, catatracts have started in both eyes which result in him not seeing very well at night, and is extremely fast to tire out. Every test possible has been done on him but they all come back negative,{thyroid-twice, blood and skin samples, urine analysis etc. Both the vets and I are at a loss as to his problem. I am the only one that feeds him and I am very strict with weighing his food exactly and he never receives any sort of treat.Max presents all the symptoms of thyroid problems but tests are negative. We even tried him on Synthroid for 3 mth but with no results. He is strictly eating Science Pro Maxi Light. I am at a loss as to what to try next. I can see that Max really wants to play and run around but he is just unable. He also had a lot of skin problems but this is over{small blessing] He is prone to very bad ear infections. Both vets are from different practices but collobrate on the best way to treat Max. Is there any way you can help. I am always afraid I will look outside and find him dead, so something has to be done, but we don't know what..When he started on Synthroid, he took 50 mg a day for 1st week and then 100 mg twice a day  2nd week and continued
until he was having 800mg twice a day.
Please Help and tell me what to try next. I await desperately for an answer
thank you in advance for your help......by the way, we live in Brasilia,Brazil where the weather is always hot and we go from dry to wet season.

Answer
I am very sorry to hear this.  I am afraid I may not be much help.  Often when somebody has a problem their vet isn't solving, I suggest a second one.  In this case, you already have 2 working on the case.  Is the Science diet you are feeding him one of the Hills Prescription Science diets?  Usually they are an excellent choice.  I had to feed the Science CD to one of my puppies and it solved her problems.  

I am sure the vets have plenty of experience with your climate and would see any problems it was causing.  

Have you talked to the breeder you got Max from?  Are other dogs from the litter having any problems?  Has anything helped them if so?

Goldens are such great dogs, especially the boys.  We are keeping a 13 year old one for friends away for the Thanksgiving Holidays here.  I have known him all his life, and we enjoy our visits.