Working dogs fed all meat diets have been reported to suffer from hypothyroidism due to iodine deficiency.

[2] Secondary hypothyroidism occurs when pituitary thyrotrophs fail to develop, resulting in impaired thyroid-stimulating hormone secretion.

[2] Secondary hypothyroidism caused by naturally acquired defects of thyrotrophs, like those resulting from a neoplasia, are uncommon.

It is not uncommon for owners to ignore certain signs of hypothyroidism due to the slow progression of metabolic symptoms.

Other dermatological symptoms include: hyperkeratosis, hyperpigmentation, seborrhoea, pyoderma, otitis externa, and myxoedema.

An immune-related cause has been put forward as well, due to the presence of immune-mediated thyroiditis in some dogs with hypothyroidism.

Hypothyroidism can result in excessive prolactin secretion due to increased thyrotropin-releasing hormone levels.

The exact mechanism behind the cause of cardiovascular and haemodynamic symptoms is unknown but may be due to the effects of hypothyroidism on the cardiac muscle.

Most cardiac abnormalities in dogs with hypothyroidism are typically mild and thus is not a cause for concern aside outside of long surgical procedures.

Thyroid hormone treatment usually results in a cessation of cardiac abnormalities, although it may take several months for this to occur.

Thyrotoxicosis is a rare side effect in dogs receiving levothyroxine treatment due to the half-life and poor absorption.

Symptoms include: polypynoea, anxiety, tachycardia, aggression, polyuria, polydipsia, polyphagia, and cachexia.

[2] Immune-mediated endocrine disorders such as diabetes mellitus and hypoadrenocoriticism often occur alongside lymphocytic thyroiditis.

[2] Given the possible comorbidities and other causes of low serum concentrations of thyroxine it can be difficult to definitively diagnose hypothyroidism.

Strong cause for a diagnosis exists when clinical signs are showing, test results are indicative, and no other illness is present.

Testing will typically reveal decreased or absent levels of thyroid-stimulating hormone; however, this result does not confirm secondary hypothyroidism.

Dogs being treated with anticonvulsant medications that lower serum thyroxine levels such as phenobarbital do not appear to cause any symptoms of hypothyroidism.

[2] Common electrocardiographic findings in dogs showing symptoms of hypothyroidism include sinus bradycardia and atrioventricular blocks.

In cases of hypothyroidism the thyroid lobes appear round or oval in shape when viewed from the transverse plane, hypoechoic, and a smaller volume and area than clinically healthy patients.

[2] A study in 2005 found an ability to diagnose hypothyroidism correctly 96% of the time when euthyroid sick syndrome was also suspected as a possible diagnosis.

Certain breeds, including the Greyhound, Whippet, Sloughi, Saluki, Basenji, Irish Wolfhound, and some Alaskan sled dogs, have levels of thyroid hormone that differ from the norm.

Thyroid hormone levels can fluctuate due to a number of causes such as another illness, especially euthyroid sick syndrome.

Levels of thyroxine and triiodothyronine in blood serum are increased in bitches during dioestrus, possibly due to progesterone improving the binding.

This method allows for normal levels of thyroid hormone in tissue and elsewhere whilst keeping the risk of iatrogenic hyperthyroidism low.

The treatment dose for dogs is 10 times that of humans due to a shorter half-life of serum thyroxine and poorer gastrointestinal absorption.

Serum concentrations of thyroxine and thyroid-stimulating hormone should be measured every 6–8 weeks or earlier if the animal does not respond well to treatment or thyrotoxicosis occurs.

While symptoms should have resolved after this 6–8 weeks full regrowth of the coat following endocrine alopecia may not occur until several months after starting treatment.

Prognosis is poor even for treated dogs with congenital hypothyroidism, many of the effects like cretinism and retarded growth result in lifelong complications such as osteoarthritis.

In addition the torpidity of the cat will cause a decrease in grooming which may lead to matting and poor coat condition.

Other causes have been reported in specific pedigrees or colonies including iodine organification and thyroid peroxidase deficiency.