Hypothyroidism in the dog
All of these tissues secrete chemical substances called hormones that have a profound effect on certain "target" tissues. But of all those glands and hormones, the thyroid gland is considered the Master Gland. If the thyroid gland is malfunctioning, every cell in the body is affected. No wonder dogs, properly diagnosed with hypothyroidism and placed on appropriate thyroid hormone supplements, will look, act and feel so much better than before treatment.
Most dogs show signs of hypothyroidism between 3 and 5 years of age but diagnosis isn't made in some dogs until they are years older. Males and females seem to be equally affected. Routine testing of young dogs is not commonly done. Some veterinarians believe that if a dog has not acquired Hypothyroidism by 5 or 6 years of age, the odds are good that it will never be a problem for that individual. (Cats rarely acquire hypothyroidism but have more trouble with hyperthyroidism, especially older cats.)
One of the most commonly seen signs that a dog may have insufficient secretion of thyroid hormone is weight gain with apparently little food intake. Any overweight pet should be checked for Hypothyroidism. In long-standing cases there often is lack of proper hair coat and even hair loss. This loss generally displays a pattern over the lumbar area on both sides, sparse hairs on the back of the rear legs and lack of hair along the abdomen. The coat tends to lack luster and the finer hairs of the undercoat may be missing entirely. Most dogs with hypothyroidism lack energy, prefer warm environments and have poor exercise tolerance. Of great concern to breeders is the fact that dogs with hypothyroidism may be infertile and many breeders have their dogs tested for thyroid function prior to breeding; in dogs with a poor breeding history, hypothyroidism often is the culprit.
If the physical exam or history indicates probable Hypothyroidism, the veterinarian will take a blood sample and have more tests run. The most common tests for thyroid function are T4 (the main Thyroid hormone) and canine TSH (Thyroid Stimulating Hormone from the Pituitary Gland). Some veterinary laboratories now recommend TgAA (Thyroglobin Auto-antibody) analysis be done because it identifies thyroiditis much earlier in the progression of the disease. All of these assays are relatively inexpensive, and the information they provide in invaluable to establishing a diagnosis.
L-thyroxine (T4) tablets are generally administered twice a day. When given twice a day, most veterinarians prescribe 0.1 mg/10 pounds twice per day as the initial dosage. Repeat exams and occasional follow-up blood testing really helps to fine tune the proper amount of medication needed for each patient.
Is hypothyroidism inherited? Evidence indicates some familial patterns of inheritance. There are more than half a dozen studies reporting the familial incidence of autoimmune thyroiditis.
Not all cases of hypothyroidism are due to autoimmune lymphocytic infiltration of the gland. There can be other "inducers" of the disease such as consumption of too much Iodine. These inducers can be very difficult to identify.
Humans may acquire what is called Hashimoto's Disease, a genetically transmitted form of hypothyroidism but this disease is not the same as autoimmune thyroiditis in dogs. In Hashimoto's Disease females are five times more likely to get the disease than males. There are other differences, as well.