Brain Tumor???

Question
OK...here are the facts: AKC Dalmatian 6 years old, not as eager with his food lately, increased thirst, decreased heart rate, occasional coughing, no neurological or personality changes. We took him to the vet today...they did bloodwork, x-rays, urinalysis, EKG - he is negative for heartworm & diabetes, positive for significant heart murmur, enlarged heart on right side, and PVCs. The vet said she thought it was a brain tumor (no way to confirm without an MRI) because of the brain not telling the heart what to do. She consulted 2 other docs and they thought of hypothyroidism. She sent us home with Pyroxine <can hardly read her writing> .3mg, we are to give him 1/2 at 6 am tomorrow and take him in at 12:15pm to make sure he is not having a reaction and also find out results from other bloodwork. I have researched brain tumors in dogs and he is not matching any of the symptoms, a link : http://www.marvistavet.com/html/body_hypothyroidism.html suggested that 26% of hypothyroid dogs experience decreased heart rate.
I wonder if he has just one or multiple things going on - hypothyroidism or brain tumor, or a heart (mitral valve) issue, or more.
Any help or direction would be greatly appreciated.  

Answer
I cannot make a link between hypothyroidism and a brain tumor.  Some hypothyroid dogs wil have seizure activity.
Did they measure the dog's thyroid levels?

If there is a heart murmur, that by itself is not serious unless there is congestion in the lungs.  PVC's can be significant or they can be benign.  In people, up to 200 PVC's per day may be normal.

I would get a second opinion on this one from someone with cardiology experience.