Hypothyroidism in Dogs

Most common in dogs between 4 and 10 years old, hypothyroidism is rare in toy and miniature breeds, more often affecting medium to large sized dogs. The most common reasons for hypothyroidism to develop are an underactive thyroid, a decrease in hormone production, and thyroid gland destruction. While easily treated, often hypothyroidism produces symptoms that can be mistaken for other conditions. Once your dog has been diagnosed with hypothyroidism, treatment can usually relieve all symptoms, and allow your dog to live a healthy life.

The thyroid gland, along with the pituitary gland, produces hormones that help to regulate the systems of the body. Hypothyroidism occurs when there is an issue with the thyroid that causes those hormones to be out of balance. The metabolism then slows down, affecting every organ, and causes various symptoms that are often passed off as old age, such as weight gain, lethargy, and coat and skin issues. Severe hypothyroidism can lead to paralysis, hypothermia and coma.

Slowed metabolism affects every body system, and can cause signs that can vary from dog to dog. They include:

• Lethargy
• Weakness
• Intolerance to cold
• Heat seeking behavior
• Weight gain without increased appetite
• Obesity
• Excessive shedding and hair loss
• Dry, dull coat
• Thin to bald coat
• Inability to grow hair after shaving or clipping
• Skin infections
• Itchy skin
• Dark skin pigmentation
• Ear infections
• High cholesterol 
• Infertility
• Lack of libido or heat periods
• Abortion
• Lameness
• Lack of coordination
• Head tilt
• Improper tear production, causing dry eye
• Fat deposits form in corneas
• Thickening of facial skin, causing a puffy appearance with drooping eyelids
• Mental dullness
• Laryngeal paralysis
• Facial nerve paralysis
• Loss of muscle coordination
• Limb paralysis
• Gastrointestinal issues
• Slowed breathing rate
• Hypothermia
• Low blood pressure
• Slow heart rate
• Coma

Congenital hypothyroidism signs include:

• Dwarfism
• Impaired mental development
• Thyroid gland enlargement

Types

• Primary Hypothyroidism – This type accounts for 95% of cases of hypothyroidism, and results from a problem with the thyroid gland; this is most commonly through lymphocytic thyroiditis, wherein the immune system attacks the thyroid, and idiopathic thyroid gland, when fat tissue replaces normal thyroid tissue
• Secondary hypothyroidism – This is usually caused by a decrease in the thyroid hormone, and a problem with the pituitary gland, often an expanding tumor that destroys the hormone producing activity of the gland
• Congenital hypothyroidism – This can be caused when the thyroid gland is underdeveloped, missing or abnormally placed, or from dwarfism, issues with iodine levels, or a hormone deficiency

The two most common causes of hypothyroidism in dogs are:

• Lymphocytic thyroiditis
• Idiopathic thyroid gland atrophy

Other causes of hypothyroidism include:

• Thyroid cancer
• Congenital hypothyroidism
• Iodine deficiency or overabundance

Signs of hypothyroidism can mimic other conditions, so a correct diagnosis needs to be made using careful attention to the symptoms and test results. Your veterinarian may run blood tests, serum chemistry panels, and a urinalysis to narrow down a diagnosis. Once hypothyroidism is suspected, a blood sample is used to measure the levels of the thyroid hormone in a total thyroxine test, or T4 concentration screening test. Low levels of thyroxine can point to hypothyroidism.

Additional tests will be performed to confirm the diagnosis, and can include further hormone testing, such as a TSH stimulation test, free T4 test, an equilibrium dialysis, and imaging of the thyroid by ultrasound, scintigraphy and sonography. The response to a thyroid hormone supplementation can help to confirm the diagnosis. Often these tests can rule out or find other causes of the hypothyroidism, such as a tumor or an iodine issue.

Hypothyroidism is not curable, but it can be treated. A thyroxine replacement is given orally for the duration of the dog’s life. This replacement starts as a standard level of the hormone, and is adjusted based on your dog’s weight. Initially, the supplement may be given twice a day, but will usually be adjusted to one time per day. A blood sample to check your dog’s hormone levels is taken after one month of treatment. Over time, your dog’s body may become tolerant of the thyroid hormone replacement, and the dose may need to be adjusted. Generally, your dog’s hormone level should be tested every six months.

Overdosing can occur, and will need to be reported to your veterinarian so that your dog’s hormone replacement level can be adjusted. Symptoms of a hormone overdose can resemble that of hyperthyroidism, and can include:

• Weight loss
• Hyperactivity
• Decreased sleep
• Increased thirst

If another cause for the hypothyroidism is found, such as cancer, treatment will be prescribed appropriately.

Within 4 to 8 weeks of treatment, you should start to see improvement in your dog. If you notice signs of hyperthyroidism, or if symptoms do not subside, talk with your veterinarian, as an adjustment may be needed. Treatment with a thyroid hormone supplement is a lifelong therapy, and your veterinarian will schedule regular check-ups to ensure your dog receives the right level of hormone supplementation throughout his life.