Dear Daisy Dog
Both my dog, Samantha, and I are hypothyroid, and both of us take thyroid hormone daily. Every year, Samantha’s veterinarian checks her T4 thyroid hormone level, while my physician checks my TSH level. Why doesn’t the vet check Samantha’s TSH level?
Hypothyroid dogs have underactive thyroid glands, which produce inadequate T4 thyroid hormone.
Thyroid hormone stimulates metabolism, so hypothyroid dogs lack energy, gain weight easily, and have coats that are dry, dull and sometimes sparse.
When the thyroid glands produce insufficient thyroid hormone, the pituitary gland increases its production of thyroid stimulating hormone (TSH). Unfortunately, even increased levels of TSH can’t persuade the failing thyroid glands to produce more thyroid hormone.
In hypothyroid humans, TSH levels remain high enough to accurately reflect thyroid function.
But in us hypothyroid dogs, they don’t. TSH levels are initially high but decrease fairly quickly and remain low, regardless of how little hormone the thyroid glands produce.
Strangely, the cells of the dog’s pituitary gland that would normally produce TSH make growth hormone instead.
Thus, veterinarians must rely on T4 thyroid hormone levels, rather than TSH, to monitor dogs receiving supplementary thyroid hormone.
Another difference between humans and dogs is the number of thyroid glands we have. A human has just one, while a dog has two.
Fortunately, hypothyroid dogs respond well to thyroid hormone in pill form, as you and Samantha have learned.