A measure of the thyroid levels in your dog determine how well the thyroid gland is functioning. When a vet tests your dog's thyroid levels, he is checking for the T3 and T4 levels, as well as for the level of thyroid stimulating hormone or TSH.
T4 is a thyroid hormone secreted to help increase the rate of cell metabolism and to regulate growth. The normal level of T4 in a dog will vary depending on the dog's age, weight, breed and health. There is no single standard for a normal T4 level among dog breeds and specimens.
Low T4 Level
If your dog has low T4 level, he may be suffering from hypothyroidism. You may have seen signs of weight gain, dry skin, coarse hair, hair loss, skin infections and lethargy. Your vet may prescribe synthetic thyroxine to supplement the thyroid hormone. You will see improvements in about three to five days.
High T4 Level
High levels of T3 and T4 levels may indicate hyperthyroidism. You may notice an increased appetite, vomiting, weight loss, increased thirst, hyperactivity, rapid or difficulty breathing and an enlarged thyroid gland -- a lump on your dog's neck. Depending on the severity, your vet may prescribe drugs to reduce the production of thyroid hormones or may opt to remove the thyroid gland.