Hypothyroid conditions ( medical conditions where the thyroid gland is not working properly, either from production of antibodies against the thyroid hormone or from the inability to produce enough thyroid hormone ) are common in dogs, while hyperthyroid conditions (medical conditions where the thyroid gland produces too much thyroid hormone, as in overmedication with thyroid hormone or from a thyroid tumor in dogs) are common in older cats.

Hypothyroidism: Common in Dogs

With hypothyroidism, the thyroid gland fails to produce a sufficient amount of thyroid hormone to meet the demands of the peripheral tissues. Hypothryoidism can affect all organ systems including digestive, skeletal, muscular, respiratory, cardiovascular, nervous, skin, pituitary, adrenals, pituitary, renal, and the blood making components of the bone marrow. The thyroid influences protein synthesis, oxygen absorption, glucose absorption and many more biological functions.
Hypothyroid dogs can be overweight, have neurological or muscular weakness in the hind legs, seizures, behavioral changes, anemia, skin and coat problems, cardiac problems, several types of digestive issues, urinary incontinence, infertility, and many other physical manifestations. Don’t surmise that your dog’s thyroid is normal because he or she is thin and has a good coat of fur.

The thyroid gland is a master gland and it regulates the efficiency of the different systems in the body. Not enough thyroid hormone can cause a slow heart rate and therefore less perfusion of the kidneys and liver. Therefore, if your pet has elevated kidney levels and poorly functioning kidneys; checking the thyroid (and if it is low, correcting it with medication) may serve to increase perfusion through the kidneys and lower the kidney values to a degree. The immune system in a dog who is hypothyroid is not working to full capacity and it opens the door for many illnesses that may have been prevented with a properly functioning immune system. Dogs with degenerative neurological weakness of the hind end are often helped with thyroid medicine if blood tests indicate hypothyroidism.
The secretory activity of this gland is controlled by feedback systems involving the pituitary gland and the hypothalamus. The synthesis of thyroid hormones takes place in the follicles of the thyroid gland and is dependent on two key nutrients: iodine and tyrosine. Other nutrients such as zinc, selenium and vitamin C are also essential for the production of thyroid hormones. However, over-supplementation with these nutrients (especially iodine) can suppress thyroid output, so a proper balance is required.

The liver synthesizes ‘thyroid transport proteins’ which move the thyroid hormone called T4 out to the peripheral tissues. The tissues then convert this into the more biologically active T3 by removing one of the iodine molecules from T4. A healthier liver helps the thyroid gland get the product out to the tissues and a healthier thyroid creates a healthier liver.

How to Detect Thyroid Problems

Interestingly, thyroid hormones increase the metabolic activities of almost all tissues in the body. This is because thyroid hormones have the capacity to activate nuclear transcription of a large number of genes, which leads to the formation of proteins, the building blocks of the body.The best blood test to analyze thyroid function is a full panel – checking for T3, Free T4, T4 and autoantibodies to the thyroid. A significant number of dogs have autoantibodies to a thyroid hormone that work actively to destroy that hormone before it can get to it’s site of action. This can confuse therapy and dosing, so it is nice to know this in the beginning. Not all laboratories do this testing. The Hemopet/Hemolife laboratory, under the direction of Dr. Jean Dodds, in California runs this full panel and Dr. Dodds reviews all blood work to make sure that it is interpreted correctly and she also advises on the dosage for medication. She is personally available for consults about thyroid problems for a small donation fee. Minimally a Free T4 and T4 should be analyzed, as if your animal has a non-thyroidal chronic illness, the T4 can be artificially reduced. Both these tests are commonly available in the United States. The T4 test is the only one easily available in New Zealand, but Massey University Laboratories can send blood serum to Michigan State University (MSU) Veterinary School for a full panel.
Once you have received the results of your pet’s thyroid test, it is important that they be interpreted correctly. This may seem like a funny thing to say but the lab values that dictate the ‘normal range’ are, in fact, not the ones you go by. Thyroid values that sit at or just above the lower limit of the lab’s reference range may be interpreted as hypothyroid, when the pet’s age and breed type are taken into account. For example: there can be a dog who is fat and greasy and flakey with a slow heart rate that has a T4 test result of 14 nmol/L when the labs reference range is 10-40 nmol/L. He is in the normal range, which is not optimal for his metabolic needs as he is clinically hypothyroid. If he was thin with a good coat, I would still interpret this result as being suboptimal and the dog in early stage hypothyroidism. This is why I like it when Dr. Dodds interprets the test. She is a world expert on thyroid problems and her interpretation can be totally trusted.

The MSU Veterinary School, routinely includes a TSH (Thyroid Stimulating Hormone) measurement along with the other thyroid levels and interprets the results conservatively without taking the age or breed of the dog into account. Furthermore, the canine TSH assay is a relatively poor predictor of primary hypothyroidism in dogs (~ 70%) than it is in people (95%). This is apparently because the dog has an alternative pathway for thyroid hormone regulatory control; current research is directed at developing improved assays for TSH in the dog. So, if you have this same fat greasy dog who is so wide you could have a dinner party for four to six people on his back and his test results come back one unit over the lower limit of the stated normal reference range, but his TSH is within normal limits, the interpretation from MSU will typically indicate that he is normal and not hypothyroid.

Dogs need a lot more thyroid medication per lb. (or kg.) than people do so their dose is about ten times higher. Dogs also metabolize thyroid medication more quickly than people do (the half-life of thyroxine in dogs is 12-16 hours, whereas it is days in people). Therefore, they need to take their medication twice a day, rather than once a day- as humans do.
It is important to have your dog’s thyroid level retested 6-8 weeks after starting the medication so your veterinarian can assess whether your companion is receiving an adequate dosage of thyroid hormone. On the day of the blood test, give your dog the thyroid pill an hour before feeding. This allows for full absorption of the pill prior to testing. You can hide it in some butter or cheese if you want. By the way, as thyroid hormone binds to calcium and soy in foods, it is always best to give the medication at least an hour before or three hours after each meal. The retesting needs to be done 4 to 6 hours after the morning pill, as it is a timed test measuring peak blood levels of thyroid hormone.

Dr. Deva Khalsa, VMD