Underactive thyroid glands or 'hypothyroidism' is the most commonly occurring form of thyroid disease in the dog. This is in sharp contrast to the cat, where hyperthyroidism (overactive thyroid glands) is more common.

Typically, dogs with hypothyroidism have a slow metabolism, which leads to a sluggish demeanor, weight gain, and lack of enthusiasm for exercise. They develop dry, brittle coats, and frequently suffer from patchy hair loss. In addition, their skin health is less robust and those affected tend to suffer from skin infections and may develop exaggerated skin folds which lend them a 'tragic' expression. 

Diagnosis is made in first opinion practice by appropriate blood tests. Thyroid supplements are widely available on prescription. The tablets are given at home, usually twice a day. The dose is assessed after four to six weeks of therapy, and monitored quarterly thereafter.

It is important to establish an accurate diagnosis. This requires a screening blood panel to identify concomitant health problems which could artificially suppress thyroid function. This is important because once these problems are treated, thyroid function may return to normal. 

When looking directly at thyroid levels, the clinician must choose which parameter to monitor. There is much debate about which is the most appropriate, but most clinicians favor either total T4 or free T4, in combination with thyroid stimulating hormone. 

Once a diagnosis is made, the patient is started on an oral thyroid hormone tablet. These are best given on an empty stomach, twice a day. After four to six weeks, follow-up blood samples check on the level of thyroid hormone in the blood, and the dose is adjusted accordingly from there. 

Most dogs respond excellently to thyroid hormone supplementation. However, hypothyroidism is vulnerable to overdiagnosis, in which case giving a supplement is unnecessary and possibly even harmful. 

Overdiagnosis results from the fact that thyroid levels can be suppressed due to other illness, hence giving a false low reading. To avoid this, the clinician is wise to look at other parameters, such as measuring the levels of thyroid stimulating hormone in the blood stream so the thyroid levels can be interpreted correctly. 

Thyroid supplements are usually needed for life. However, some dogs can be switched from twice daily to once daily dosing, once they have responded to the initial loading doses. 

Absorption of thyroid hormone supplements is enhanced when the medication is given on an empty stomach. 

An improvement in the dog's demeanor and coat condition is usually seen within four to six weeks of starting therapy. Monitoring tests check on the level of thyroid hormone in the blood stream and dosage adjustments are made from there. 

Supplementation is required for life, and the veterinarian must be consulted before changing brands of thyroid supplement, because not all are interchangeable and there is a risk of destabilization. 

Hypothyroidism is typically diagnosed and monitored in first opinion practice. The patient requires regular check-ups two to four times a year. This may involve blood tests costing anywhere from $60 to $150. The medication itself costs approximately 50 cents a day, bearing in mind this is for life. 

When a patient is accurately diagnosed, there are few risks or side effects associated with thyroid hormone supplementation. This is because it is a lack of hormone which is being supplemented, hence bringing the body back into balance. 

However, it is important to check that the dose is neither too low, or too high. Where the dose is too low the dog's overall quality of life is reduced. Where the dose is too high there is a risk of increasing the metabolic rate and putting damaging pressure on the heart, kidneys, and vital organs. 

Some breeds of dog, such as the Siberian husky, are genetically predisposed towards hypothyroidism. Unfortunately, it is difficult to eradicate this condition by selective breeding, because hypothyroidism often develops at an age when the breeding stock have already produced litters. Screening before the condition develops will show a 'normal' result, as the markers in the blood that damage the thyroid, have not yet developed. 

However, dogs that are known to suffer from hypothyroidism should not be subsequently used for breeding. 

Many cases of hypothyroidism are 'idiopathic', meaning no identifiable cause is found. In these cases, preventative therapy is not possible.