Is my ferret molting or can he have adrenal disease?

i have had my ferret for almost 3 years and every year he tends to lose alot of fur around summer time, it usually grows back during winter, but this year he is losing alot more then usual, his tail is still furry but most of his back is losing fur but small peach fuzz is covering the bald spots. he is very heavy in weight and is very active and eats and uses the bathroom with no problem.