Hypothyroidism in Dogs

What is the thyroid gland?

The thyroid has two lobes which sit on either side of the trachea (breathing pipe) on the underside of a dogs neck. The thyroid controls a dog’s metabolism.  If the gland is over active it is called hyperthyroidism.  Hyperthyroidism is extremely rare in dogs.  If the gland is under active it is called hypothyroidism, which is relatively common in older dogs. Hypothyroidism is caused by either immune mediated destruction of the thyroid gland, called immune mediated lymphocytic thyroiditis.  Or it is caused by idiopathic thyroid atrophy, where the thyroid tissue is replaced with fatty tissue.  Immune mediated destruction and idiopathic thyroid atrophy account for more than 90% of the cases of hypothyroidism.

Clinical Signs of Hypothyroidism

• Weight gain without an increase in calorie consumption

• Lethargy, lack of energy

• Cold intolerance

• Skin and fur abnormalities

• Symmetrical hair loss

• Dull dry hair coat

• Hair that does not regrow after it is shaved

• Dandruff

• Recurrent skin infections

• Increased pigmentation to skin 
  

How do you diagnose hypothyroidism?

• Relevant clinical signs

• Thyroid hormone testing

• Total thyroxine level (TT4): this is the initial screening test.  If the TT4 is low then looking at free T4 by equilibrium dialysis is the next step, which if low confirms a definitive diagnosis of hypothyroidism.  

How is hypothyroidism treated?

A cure for hypothyroidism is not possible.  Treatment includes oral daily, lifelong supplementation of thyroid hormone. The synthetic thyroid hormone most commonly used is called levothyroxine.  Levothyroxine needs to be given orally every 12 hours. 

What is the prognosis for hypothyroidism?

The prognosis with proper diagnosis and treatment is good.  Many of the abnormal clinical signs will resolve within a few months of starting medication.