A while back we talked about sick euthyroid syndrome – a condition that complicates the diagnosis of hypothyroidism in dogs. But once it has been determined that a dog truly has hypothyroidism, the question of what the thyroid gland does and how the body is affected by its dysfunction must be answered.
|Thyroid gland: Image Pets Adviser|
What does the gland do?
The thyroid gland is located in a dog’s neck, one segment on either side of the trachea (windpipe) and makes hormones – primarily thyroxine (T4), but also 3,5,3’-triiodo-thyronine (T3), reverse T3, and other metabolites. T3 is the more potent hormone and is not only produced directly by the thyroid gland, but is also derived from T4. These hormones are made from the amino acid tyrosine and are bound to iodine (this is the only place iodine is utilized in the body).
Thyroid hormone secretion is regulated by a negative feedback mechanism of the hypothalamic-pituitary-thyroid axis (from the brain to the thyroid). This means that when there is a decrease in T4, more T4 is made. Conversely, when there is an excess amount of T4, less is made.
What do the hormones do?
On a molecular level, thyroid hormones bind cellular receptors (intracellular DNA-binding proteins), enter into cells via membrane transporter proteins and interact with specific sequences of DNA to modulate gene expression – in other words, turn some genes on and other genes off.
On a physiological level, thyroid hormones have numerous effects in many, if not, all areas of the body. While their absence or excess may not be life threatening (well, not immediately anyway), maintaining appropriate levels is certainly important for good quality of life. Some known effects of thyroid hormones include:
- Metabolism – thyroid hormones help to regulate body temperature, and can stimulate fat and carbohydrate metabolism. This results in increased fatty acids and glucose in the blood. When the hormone is deficient, as is the case in a hypothyroid dog, the blood cholesterol and triglyceride levels become elevated.
- Growth and brain development – normal growth and brain development in young animals is dependent on thyroid hormones. A thyroid deficiency results in growth-retardation and neurologic impairment.
- Cardiovascular effects – thyroid hormones cause an increased heart rate, cardiac output, and blood flow to many organs.
- Central Nervous System – thyroid hormones cause agitation and anxiety in excess, and sluggishness and dullness if deficient.
- Reproductive System – fertility is affected by lack of thyroid hormone
- Many other effects - such as maintaining healthy skin and good muscle tone
You can see why having appropriate amounts of thyroid hormones is critical to the well being of dogs.
Thankfully, treating hypothyroidism in dogs is about as easy as it gets. Simply give synthetic thyroid hormone as prescribed by your veterinarian and follow the monitoring schedule that he or she recommends (it can take some time to find the right dose and a dog’s needs may change).
If despite this, your dog’s condition does not improve, the initial diagnosis of true hypothyroidism versus euthyroid sick syndrome needs to be revisited.
Dictionary of Veterinary Terms: Vet-speak Deciphered for the Non-veterinarian.
Dr. Coates has recently joined the PetMD team and she is now writing for the Fully Vetted column; great blog, do check it out.
Jennifer also writes short stories that focus on the strength and importance of the human-animal bond and freelance articles relating to a variety of animal care and veterinary topics. Dr. Coates lives in Fort Collins, Colorado with her husband, daughter, and pets.
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