What is thyroid disease in pet animals?
Your veterinarian may have recently discussed thyroid disease in your dog or cat with you. In dogs, low levels of thyroid hormone, or hypothyroidism, tends to affect middle-aged dogs, and we frequently screen for it on yearly or pre-anesthetic bloodwork. Cats, especially older kitties in their teens, are predisposed to the opposite problem, developing high levels of thyroid hormone, often secondary to a benign hormone secreting thyroid tumor. Managing these diseases is usually done through oral medication and is often successful in bringing thyroid hormone levels in to their desired normal ranges and controlling the symptoms of the disease.
Why is thyroid disease important?
Thyroid hormones affect a huge number of systems in the body. During growth and development, thyroid hormones play an essential role in normal formation of the neurologic and skeletal systems. Congenitally hypothyroid puppies often show very stunted growth patterns. Thyroid hormones in adults function in increasing metabolism in the tissues, increasing the heart rate, breaking down fat, stimulating red blood cell production, and regulating cholesterol. These functions are all affected when thyroid hormones are too low or too high, which results in the symptoms that we see in our patients.
Facts about canine hypothyroidism:
Hypothyroid dogs are usually diagnosed during middle age, on average about 7 years old. About 50% of the time it is caused by a condition called lymphocytic thyroiditis, an immune-mediated condition where the body starts to create antibodies to the thyroid tissue. Other causes can be due to a cancer of the thyroid gland, a secondary condition involving the pituitary, or for unknown reasons, what we call “idiopathic.
Symptoms of hypothyroidism include:
- Strong appetite and obesity not responsive to diet or exercise therapy
- Fatty deposits in the cornea of the eye
- Neurologic abnormalities
- Vestibular disease
- Peripheral neuropathy
- Dermatologic problems
- Hair loss, especially symmetrical hair loss on both sides
- Poor re-growth of hair after shaving or clipping
- Dry and flaky or very oily skin
- Recurrent skin infections
Diagnosis of canine hypothyroidism
We diagnose canine hypothyroidism through a combination of symptoms, physical examination and blood tests to check for circulating hormone levels.
The thyroid glands are located in the neck just behind the larynx (voice box) in dogs and cats, and actively produce thyroid hormones, including thyroxine (also called T4). Production of thyroid hormones is regulated by the pituitary gland, through a hormone called thyroid stimulating hormone (TSH). A feedback loop exists between the body and the pituitary gland, with TSH production by the pituitary going up when the body needs thyroid hormone and turning off when thyroid hormone levels are high.
Screening the blood for indicators of hypothyroidism may include the following tests:
- Complete blood count – may show a mild anemia in hypothyroid dogs
- Serum biochemistry – often shows high levels of fasting triglycerides and cholesterol with canine hypothyroidism
- TT4 – evaluates total T4 concentration in the blood
- Free T4 (fT4) – evaluates only the portion of circulating T4 that is not bound to protein in the blood.
- This is a more sensitive indicator of disease. Some dogs that are not truly hypothyroid may have a low TT4 but a normal fT4.
- TSH levels – may be increased as the pituitary produces more and more TSH to try to stimulate an under-responsive thyroid gland
- Anti-thyroid antibody – a test that may help determine if autoimmune disease is present
Certain kinds of drugs (e.g. sulfa antibiotics, anti-inflammatory, anti-depressant and anti-seizure medication) can cause artificially lowered thyroid levels so it is important to make sure we account for these before making a diagnosis. In addition, some breeds, such as greyhounds, will normally have lower TT4 levels.
Some non-thyroid diseases can also cause circulating thyroid hormone levels in the blood to decrease. Your veterinarian may need to do other tests to rule these diseases out before confirming a diagnosis of hypothyroidism.
- Diabetes mellitus
- Adrenal disease
- Hypoadrenocorticism (Addison’s disease)
- Hyperadrenocorticism (Cushing’s disease)
- Renal disease
- Heart failure
- Severe infections
Treatment of canine hypothyroidism
Treatment for hypothyroidism involves lifelong oral medication with levothyroxine, a relatively inexpensive thyroid hormone supplement. Generally blood tests are rechecked approximately 4-8 weeks after starting medication and again as needed while the dog’s metabolism adjusts to the therapy. After that, once yearly checks are adequate to ensure that the thyroid hormone level remains in the normal range. The prognosis is usually very good for long-term management, but complete resolution of symptoms may take several months.
Next blog from Dr. Rainwater: Feline hyperthyroidism!