Hypothyroidism in dogs occurs when there are decreased levels of thyroid hormones present within the body. It is common in middle-aged to older dogs. These hormones are important for maintaining heart rate, a healthy hair coat, energy levels, ideal body condition, and normal body temperature.
What causes it?
The pituitary gland secretes thyroid stimulating hormone (TSH) which stimulates the thyroid gland to produce T3 and T4 (thyroid hormones). When there is a problem along this pathway, low levels of thyroid hormone can occur. The most common cause of hypothyroidism is lymphocytic thyroiditis, which means that the immune system is attacking the thyroid gland and causing inflammation and destruction. Atrophy of the thyroid gland may also occur as dogs age. A tumor within the pituitary gland that prevents TSH from being produced is another possible cause, but it is much less common than primary thyroid disease.
What are the clinical signs?
Since thyroid hormones are important for metabolism, dogs typically experience weight gain despite a normal diet. Hypothyroid dogs will often have fat accumulating along their hind ends, shoulders, or neck regions. Low heart rate, lethargy, and abnormal hair coat (shedding, hairless areas, dandruff, thickened skin) are often found. Hypothyroid dogs also cannot regulate their body temperature as efficiently and are often heat-seekers.
How is it diagnosed?
Often, this disease is detected through annual wellness bloodwork when T4 levels are checked. If results indicate a low T4, hypothyroidism should be considered. However, many non-thyroidal illnesses can cause low T4 levels so it is important to decipher these based on clinical signs and additional testing. In dogs where hypothyroidism is suspected due to a low T4, a comprehensive thyroid panel is submitted. If the values in this panel match what we would expect to see in a hypothyroid dog, your veterinarian can make the diagnosis. High cholesterol and high triglyceride levels are often found on bloodwork as well.
What is the treatment?
Treatment consists of supplementation of thyroxine (T4). This is a relatively inexpensive medication that is given orally every day. Hypothyroid dogs must be regularly monitored while receiving this medication to ensure the dosing is appropriate to maintain normal thyroid hormone levels.
What is the prognosis?
The prognosis is generally good for dogs receiving supplementation. As long as regular monitoring is performed via bloodwork and veterinary visits, most dogs can go on to live normal lives.