Hypothyroidism, or an underactive thyroid gland, is common in dogs, however most dogs with hypothyroidism develop the condition in adulthood. Congenital hypothyroidism (an underactive thyroid gland at under one year of age) is rare in puppies. Most of the puppies experiencing hypothyroidism have an underdeveloped thyroid gland, as opposed to a normal thyroid gland that fails. The thyroid gland mixes tyrosine with iodine to make thyroid hormone. Your dog’s pituitary gland produces thyroid stimulating hormone (TSH) which will prompt the thyroid gland to make thyroid hormone (thyroxine) which will control the metabolic and activity levels in your dog. A problem with the thyroid gland or pituitary gland can lead to hypothyroidism.

Congenital hypothyroidism in puppies is due to a thyroid gland that is inactive or less active than normal prior to the puppy being one year old.

Should your puppy have hypothyroidism, he will likely fail to grow at the typical rate of puppies of the same breed. His legs may be shorter than is normal and it is common for dogs to experience atypical bone development in their skull. The tongue and eyeballs of dogs with hypothyroidism often protrude, leading to a “bug-eyed” look and your puppy may have a goiter. Puppies with hypothyroidism will likely be underdeveloped physically and mentally. Other symptoms include:

Types 

Congenital hypothyroidism in dogs can present as congenital primary hypothyroidism or congenital secondary hypothyroidism. Congenital primary hypothyroidism is due to an issue with the thyroid gland, while congenital secondary hypothyroidism is due to something wrong with the pituitary gland.

Congenital hypothyroidism can be primary or secondary. Congenital primary hypothyroidism is caused by an underdeveloped thyroid gland, iodine deficiency or problems in the organification of iodine. Congenital secondary hypothyroidism can be caused by a deficiency in the thyroid stimulating hormone (TSH) or pituitary dwarfism. The deficiency in TSH varies and the clinical signs that are exhibited are often caused by a deficiency in growth hormone as opposed to thyroid hormone.

Should you notice that something appears to be not quite right with your young puppy, you will want to bring him to your veterinarian. The veterinarian will first conduct a physical examination of your puppy. He will also ask you for detailed information regarding what symptoms your dog has displayed, as well as when the symptoms appeared. Depending on the exam and information you provide, the veterinarian will likely choose to conduct a blood test to check the levels T4 and thyroid stimulating hormone (TSH). The results of this test will typically indicate whether your dog has hypothyroidism, as the disease will cause lower levels of thyroxine. A thyroid biopsy and histopathology are often required in order for your veterinarian to make a definite diagnosis. Ultrasonography and scintigraphy of the thyroid gland are other options for determining if your dog has hypothyroidism, as a radiologist with experience can use these to tell whether the dog is experiencing hypothyroidism or euthyroid sick syndrome.

Treatment of congenital hypothyroidism in puppies will include supplementing thyroxine (the thyroid hormone) with synthetic levothyroxine (also known as T4). This is available as a tablet and a liquid and will be taken one to two times each day. In dogs that do not improve with levothyroxine, likely due to poor absorption, your veterinarian may try Liothyronine sodium (T3), which will be given three times per day.

Ideally treatment will begin as early as three weeks of age, as that will lead to the best opportunity for success. As it is not simple to identify the condition in a newborn dog, starting treatment that early can be a challenge.

You will want to follow the recommendations of your veterinarian regarding follow up appointments to ensure the best outcome for your dog. Once your dog is receiving synthetic levothyroxine, your veterinarian will likely seek to check his levels three to four weeks after his treatment has begun. Your veterinarian will also ask you about any changes in your dog’s symptoms since he has begun treatment. Depending on the results of the test of your dog’s thyroid hormone, your veterinarian may choose to increase his medication. Should your dog not respond to levothyroxine, your veterinarian may decide to try Liothyronine sodium (T3). Should that not be effective, your veterinarian will reevaluate your dog to determine if the initial diagnosis is correct, and/or if there is another condition that requires treatment.