Yung-Tsun Lo D.V.M., Ph.D of Bioguard Corporation explains all there is to know about the most common hormonal (endocrine) disorder in dogs.
Dogs have a thyroid gland made up of two lobes, sitting right at the front of the neck along the windpipe. The thyroid’s main job is to produce hormones that keep the metabolism running properly, which helps all the organs do their work. When the gland doesn’t make enough of those hormones, we call it hypothyroidism. It’s actually pretty common in dogs, especially those between 4 and 10 years old, and it tends to show up more often in medium to large‑breed dogs. With too little thyroid hormone, the metabolism slows down, and that’s when you start seeing changes in the dog’s health and energy.
What causes hypothyroidism?
When we talk about the causes of hypothyroidism in dogs, most cases—around 90%—are what we call primary hypothyroidism. That usually comes from one of two things. The first is lymphocytic thyroiditis, where the dog’s immune system mistakenly attacks the thyroid tissue, and there’s often a hereditary link. The second is idiopathic thyroid atrophy, where the thyroid tissue is replaced by fat without any inflammation.
Secondary hypothyroidism is much less common. When it does happen, it can be linked to things like thyroid cancer, pituitary gland disease, congenital defects, or even external trauma that interferes with normal hormone production.
What are the risk factors?
Hypothyroidism in dogs most often shows up in middle‑aged animals, usually between 4 and 10 years old, with the average being around 6 to 7. It’s seen more often in spayed females and neutered males. Certain breeds also seem to be more prone to it—Golden Retrievers and Doberman Pinschers are classic examples.
What are the signs of hypothyroidism?
- Lethargy and reduced activity
- Weight gain
- Hair loss (alopecia), poor coat quality
- Cold intolerance
- Neurological signs in severe cases
- (weakness, nerve dysfunction)
- Slow heart rate
- Increased susceptibility and occurrence
- of skin and ear infections
- Fat deposits in the corneas of the eyes
How is hypothyroidism diagnosed in dogs?
When a dog’s metabolic rate slows down, you’ll often see changes across the whole body. Routine lab work can sometimes pick up clues, like mild anemia or elevated cholesterol and kidney values. But the only way to make a definitive diagnosis is by checking thyroid hormone levels directly in the blood:
- Blood tests measuring total T4, free T4, total T3,
- and TSH levels.
- Rule out non-thyroidal illness that can
- mimic hypothyroidism.
How is hypothyroidism treated?
The good news is that hypothyroidism can be managed very effectively. Treatment usually means giving the dog synthetic thyroid hormone—levothyroxine—every day for the rest of their life. It’s a simple pill, and most dogs respond really well to it.
Once your dog starts thyroid medication, your veterinarian will usually want to check back in after a few weeks. That follow‑up includes another exam and blood tests to see how well the treatment is working. Because hypothyroidism requires lifelong management, regular checkups and blood tests will continue throughout your dog’s life to make sure the hormone levels stay in balance.
What’s the prognosis for dogs with hypothyroidism?
With the right diagnosis, treatment, and ongoing monitoring, hypothyroidism in dogs carries an excellent prognosis. Most dogs do very well on daily thyroid medication, regaining energy, improving coat quality, and living a normal lifespan— as long as the treatment and checkups stay consistent.
