Hypothyroidism occurs when there is impaired production and secretion of thyroid hormones from the thyroid glands. This results in a decreased metabolic rate, and symptoms that are associated with this. It’s most common in dogs, developing rarely in other species such as cats and horses, so here we will be focusing on dogs. 

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The signs of this condition are vague. This vagueness of signs in addition to challenges in interpreting blood results can make it tricky to be certain of a diagnosis. This could lead to dogs being diagnosed with the condition when they in fact don’t have it. We will talk more about this later. Once diagnosed, lifelong medication usually successfully manages the signs. Let’s take a deeper look at this tricky condition.  

What causes it?

The thyroid glands sit in the neck, and produce thyroid hormones affecting the whole body, and altering the metabolism. Hypothyroidism occurs when these glands become underactive. The opposite condition, hyperthyroidism, is where the glands become over-active which is rare in dogs. Hypothyroidism can affect any sex, age, or breed of dog, but is most common in mid-large breeds of 4-10 years of age. Neutered females may have a slightly higher risk than un-neutered females. 

95% of the time this condition is caused by destruction of the thyroid gland tissue itself. In rare cases a tumour may be responsible, but this often causes slightly different signs. Very rarely dogs may be born without a functioning or developed thyroid, or the disease occurs as a secondary consequence to another condition. 

Could my dog have hyperthyroidism?

The signs of this condition can be vague, mainly relating to a slowing metabolism. Dogs may have a slow heart rate, known as bradycardia. Commonly they will show sluggishness, lack the desire to go on walks, gain weight without really eating more, and seek heat. 

Coat changes are common. Hair thinning often occurs on both flanks of the dog, or on the tail (known as ‘rat tail’). The skin is often dry and may be more pigmented. Skin thickening around skin folds, especially around the face, can give them a sad face.

The condition can affect fertility in males and females. Deficiencies of thyroid hormones within growing pups in the uterus can lead to pups that are stunted in size and mental ability.

More rarely, the disease can lead to neurological disorders such as generalised weakness, wobbliness, reduced reflexes, and specific nerve paralysis around the face and neck. A condition called megaoesophagus, where swallowing becomes difficult can rarely occur, although the cause of this is uncertain. 

An extremely rare and serious form of the disease known as myxoedema coma can occur, causing collapse and a dangerously low temperature. Cases like this are exceedingly rare.  

What can my vet do to diagnose it?

As the symptoms can be vague, and may be similar to many other diseases, signs alone cannot diagnose the condition.

Blood tests looking for a reduced level of the thyroid hormone thyroxine (T4) can be done. However other illnesses can cause thyroid hormones to drop. This is sometimes referred to as ‘sick euthyroid’ and can give a false impression of suffering hypothyroidism. Other conditions must be ruled out or taken into consideration when interpreting a low thyroid level result.

Some medications can affect levels and certain breeds may have naturally lower thyroid levels. If the T4 is normal this usually would rule out thyroid disease. If the levels are low and there are signs, further tests are needed. FreeT4 (fT4) is a form of the hormone less likely to be affected by other illness. Thyroid stimulating hormone (TSH), responsible for stimulating thyroid hormone production, would usually be high in affected dogs. If thyroid levels are low the body should be working in overdrive producing TSH in an attempt to bring thyroid levels back up.

So, if T4 is low, TSH is high, and the pet has symptoms, the condition is confirmed. To complicate things, though, a normal TSH can occur in around 40% of hypothyroid dogs… So only a high result is helpful. There is a further test for cases where the TSH is normal, but the vet is still suspicious. The test is not easy to source, so often a trial on treatment is the best option in this situation. 

Sometimes imaging can be helpful to look at the glands themselves, but often specialist equipment is needed, which is not widely available. 

Blood and urine tests to check for other conditions, and to look for changes you may see with thyroid disease, are usually also performed. 75% of affected dogs have high cholesterol and up to 50% are anaemic. These results alone do not diagnose the condition, but like a part of a giant puzzle they may increase suspicion.

What treatments are available?

Treatment involves replacing the thyroxine hormone with a synthetic version, L-thyroxine. It’s often started twice daily and reduced after a period to daily dosing. It’s ideally given on an empty stomach, although if this is not possible, it may just lead to the dose being adjusted. Often mental alertness will be the first sign to improve. Skin changes can take several months to improve. Neurological signs such as nerve damage may improve quickly, but usually take several months to completely resolve. 

Success can be measured by response as well as blood tests to check levels. Once stable, blood tests are usually done intermittently to check levels. Medication is life-long. If there is no response, it must be considered that the diagnosis of hypothyroidism may be wrong. 

Lower medication doses may be needed in dogs with certain heart diseases and other hormone conditions, such as Cushing’s disease, may complicate treatment. 

In exceedingly rare cases when dogs become seriously ill with myxoedema coma, much more intensive inpatient treatment is needed, and the outlook is not as good. 

The outlook for most dogs with hypothyroidism is great with treatment. Usually the clinical signs do resolve giving a much better quality of life, and a normal life-expectancy.

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