Thyroid Disease in the Dog

The thyroid gland performs one essential function. It sets the body’s metabolic rate. Thus, it affects every cell in the body. Thyroid hormone and vitamin D are the only two substances that do this. If the body is cold, the thyroid gland releases hormones and increases metabolism. If it is too hot, the thyroid gland stops releasing hormones. If the body is under stress, the thyroid gland reduces the release of hormones, so that cells do not self-destruct.
The thyroid system consists of the hypothalamus, the pituitary, and the thyroid. The hypothalamus senses changes in body temperature. If it is too low, the hypothalamus releases a thyroid-releasing hormone that stimulates the pituitary. The pituitary then releases a thyroid-stimulating hormone which causes the thyroid to produce more hormone. The system is quite complex and malfunction in any part will produce thyroid disease. In the dog, thyroid disease is almost always hypothyroidism. Interestingly, in cats thyroid disease is almost always hyperthyroidism.
There are three forms of hypothyroidism- primary, secondary, and tertiary. Primary hypothyroidism occurs when the thyroid itself fails to operate properly. One common cause of primary hypothyroidism is excess fluoride in the body. Fluoride takes the place of iodine in the synthesis of thyroid hormone. This results in the production of an ineffective hormone. It is easy to measure fluoride in the body by a urine test. I have tested several dogs for fluoride. So far, all the dogs which were being fed commercial dog food had elevated fluoride- and insufficient iodine. Dogs I have tested that were fed home-prepared food all had normal fluoride and iodine levels. It appears that the excess fluoride comes from food and not drinking water. It is speculated that the excess fluoride comes from bone meal which is used in commercial dog foods.
Another common cause of hypothyroidism is autoimmune thyroiditis- Hashimoto’s disease. In Hashimoto’s, the body produces antibodies that destroy thyroid cells. This condition must be treated as an autoimmune disease. It is not in the patient’s best interest to merely give thyroid medication. Patients with an autoimmune disease have an 80% chance of developing another autoimmune disease.
Under conversion of the inactive form of thyroid hormone (T4) to the active form (T3) is a common cause of hypothyroidism. Several nutrient deficiencies may be involved, but selenium is the most likely. Chronic inflammation will also interfere with conversion. The elevated cortisol associated with chronic inflammation blocks conversion. Elevated cortisol also reduces the permeability of cells to thyroid hormone.
In addition to primary hypothyroidism, a secondary and a tertiary form are occasionally seen. Secondary hypothyroidism is caused by a pituitary malfunction. The tertiary form occurs when the hypothalamus fails. These forms are relatively rare compared to primary hypothyroidism.
Hypothyroidism produces a myriad of symptoms. The most common in dogs are cold intolerance and poor hair coat. It can also cause sore muscles, muscle cramps, constipation, cold feet and ears, low body temperature, itchy skin, slow wound healing, fatigue associated with an increased need to sleep, and chronic digestive conditions, especially low stomach acid.
Because thyroid disease is so complex, it is important that proper tests be run to diagnose it. Simply running serum T4 and/or T3 will miss many cases of hypothyroidism. It is in the patient’s best interest to run a complete thyroid profile. This should include T4, free T4, T3, free T3, TSH, and anti-thyroid antibodies. A complete profile will ensure that serious underlying conditions are identified.
Treatment of hypothyroidism can be as simple as giving only T4 supplementation. However, if autoimmune disease is suspected, treatment will be much more complicated. There are several types of thyroid medication. The most common is l-thyroxine (T4). Sometimes, as in cases of under conversion, liothyronine (T3) is used. Finally, there are natural forms of thyroid medication that contain both T4 and T3.
Thyroid disease is so common and so serious that I recommend thyroid testing at a younger age than is commonly done. A thyroid profile gives one a glimpse into the adequacy of metabolism and the nutritional adequacy of the dog. It is much better to make this assessment early when treatment is much easier.