mallow

My son is 15 years old. He has been having some wierd problems, and I would like some help. My son is overweight, he weighs around 245 pounds and is about 5'10" tall. Although he is still overweight, he has lost around 30 pounds since October, he also has a pretty regular excercise plan. My son has a hypothyroid, which he takes 100MCG Synthroid medication for every morning, he also takes Phentermine, a pill which was perscribed to help him lose weight. Until now, he has had no problems, although recently he has had a high heart rate around 100-80 (at rest).Three days ago, he began to complain about slight shallow breathing, he said it wasn't that bad so we kind of ignored it. Just yesterday, Monday, he went to work out with his personal trainer and there seemed to be no problems, but around 2AM he began to experience wierd symptoms that resembled a heart attack. He said when he lied down to sleep that night, he felt really anxious and he could feel and hear his heart beating. He also said his fingers were cold and his left palm was sweaty. Although all of this he said there was little to none chest pain. He said he turned the TV back on and calmed himself down a bit and the anxiousness and beating stopped. Could this be an anxiety attack (which he has never had) or something more? He could have been comparing his shallow breathing to a story he heard about one of my friends having a Sarcoma which started out with shortness of breath. Please help me with this, for I do not know what is wrong. He is schedualed to see a Cardiologist on Wednesday, but any help prior to that would be excellent.

Thanks in Advance.