Hello, my name is Dr. Kara and I have over 20 years of experience as a veterinarian.
Hypothyroidism or low thyroid function in dogs can lead to symptoms of lethargy, weight gain, poor coat and hair condition, especially hair loss on the tail (rat tail) and collar area, secondary skin and other infections, rarely it can lead to abnormal neurologic function such as loss of feeling, incoordination, and behavioral changes.
Things that you cannot see but happen internally are anemia (low numbers of red blood cells), increased cholesterol, and changes in heart rhythm.
We can easily diagnose the condition by a blood test called a full thyroid profile. A T-4 alone is not a reliable test.
We do see fatty tumors or lipomas with some frequency in dogs, especially as they age. These tumors tend to be benign. We can get an idea of whether this is a lipoma by taking an aspirate of the mass and looking at it under the microscope, but a more definitive diagnosis is achieved by a collecting a biopsy. We recommend removal if they are growing quickly or are in an area that will interfere with his ability to get around, such as around the legs.
Please let me know if you have any further questions.