Boxer Hypothyroidism

Question
I have a 9 yr. old, neutered, boxer.  After three consecutive visits to the vet for lethargy, excessive weight gain, and sporadic seizure like activity after excitement, we were told he possibly had cancerous tumors.  With more research, I found that he possibly had hypothyroidism.  With other symptoms, like alopecia, skin pigment changes, disorientation, severity to changes in temperatures, etc., with a blood test for T4 and T3 to verify, this condition is easily treatable with medication and blood work.  
I have had Herc on 8mg of L-Thyroxine for over 45 days now.  After the first 28 days, there was a remarkable difference in his behavior and weight loss.  The clinic did the blood test again, and said that he was within "normal" limits for the thyroid levels, but didn't want to lower his dosage for another 3 months.  I called regarding him "excessively exfoliating" skin, which they said was normal, but now, he's changing hair color, dramatically, as well.  He should be a "red" colored boxer, but, he's washed out to a "bleach blonde".  Is this due to the medications?  Should I be giving him some supplements?  I respect my vet's office, but, I still have uneasy feelings about the previous misdiagnosis.  Should I be concerned?  Should I insist on another blood test?  Ask for a second opinion?  I would appreciate your insight into this situation.
Thank You,
Debbie  

Answer
We have seen thyroid medicine change the color of the coat, but this is reversible once the thyroid is under control.  His skin will go through some changes also but again, in the end it should all level out.

I can see how the vet gave you a cancer diagnosis as weight gain is the norm with hypothyroidism, not weight loss.

Weight loss is often a signalment of cancer. Boxers are prone to tumors so coupled with the weight loss it is easy to think of cancer first.

Instead of supplements get him on some really premium food such as Science Diet which is formulated for optimal nutrition. That way it can help his body get better along with the medications.

Supplements are not needed with good food like SD.