Hypothyroidism

What is hypothyroidism? 

The thyroid gland is one of the endocrine glands, ie they produce hormones – chemicals that move around through the blood stream and affect almost every aspect of life, from reproduction and growth, through to food metabolism and stress. The thyroid gland is particularly involved in metabolism, and thyroid hormone affects almost all tissues in the body. This endocrine system is controlled by the pituitary gland, which sits just under the brain and connects it with the rest of the hormone system. The pituitary gland controls the workings of all these hormone glands by a system of feedback. So the pituitary gland produces thyroid-stimulating hormone (TSH), which circulates through the blood until it reaches the thyroid glands in the neck, where it tells them to produce thyroid hormone. In simple terms, this is produced in two forms – T3 (triiodothyronine, the active form) and T4 (thyroxine, the precursor hormone, some of which will be converted to T3). The thyroid hormone flows around the bloodstream, with the T3 acting on a range of tissues, until it gets back to the pituitary, where it causes the pituitary gland to tell the thyroid glands to produce more, or less, thyroid hormone to keep the system in balance.

Normally the endocrine system works well, but occasionally things go wrong, and one of these can be when the thyroid gland is not producing enough thyroid hormone. This is hypothyroidism (not to be confused with hyperthyroidism, where there is too much thyroid hormone). Hypothyroidism in dogs is usually caused by damage to the tissue of the thyroid gland. This can be caused by the dog’s own immune system attacking it (autoimmune hypothyroidism) or by atrophy (wasting away) of the gland itself. It is thought that the latter may actually be a result of earlier autoimmune attack. There are other potential causes, including pituitary malfunction and congenital hypothyroidism, but these are thought to be rare in dogs.

The dobermann is one of several breeds of dog where hypothyroidism is prevalent. 

What are the symptoms?

Symptoms usually start showing in ‘middle age’ – about five years onwards. A genetic marker has been identified that is twice as likely to be present in hypothyroid dobes, but because it can also be present in ones with normal thyroid function, it is not a useful test on its own. It is possible to measure antibodies against the thyroid gland proteins from which thyroid hormones are made (TgAA test), which would indicate if a dog is at risk of developing autoimmune hypothyroidism later in life, but note that only about 20% of dogs that test positive will go on to become hypothyroid. Also, a negative TgAA test is only applicable at that point; the dog could go on to develop antibodies at any later time.

As with many endocrine diseases, the symptoms of hypothyroidism can be confused with a range of other illnesses, so although the symptoms below are common, they do not, of themselves, indicate that the dog is hypothyroid. Symptoms include:

> decreased appetite

> weight gain despite normal levels of feeding

> hair loss – often loss of coat on flanks, and a ‘rat’s tail’ can be a noticeable symptom

> feeling cold – hogging the fire even when it’s not very cold

> recurring skin infections – dogs can be scaly and also smelly, due to excessive oil production

> lethargy or listlessness, unwilingess to exercise

> anaemia

> slow heart rate

> nerve disorders, including facial paralysis, head tilt, muscle wastage and stiffness

> constipation, vomiting or diarrhoea

What tests are carried out to diagnose it?

The vet will carry out one or more tests to achieve an accurate diagnosis. This is not straightforward.

> Measuring levels of T3 is not helpful on its own because levels fluctuate and can be in the normal range for dogs that are hypothyroid. 

> Measuring total T4 (TT4) levels can be useful, but unfortunately this can be affected by other illnesses or by drugs the dog is taking, including steroid replacement and anti-inflammatories. A normal result means the dog is unlikely to be hypothyroid, but a positive result does not necessarily mean it is hypothyroid. Therefore, if the total T4 result is positive, it needs to be backed up with a TSH test. 

> Measuring the free T4 (the very small proportion of the T4 that is available to be converted to T3; about 1/1000 of total T4) by equilibrium dialysis (ED) is probably the most accurate test. 

How is it treated and what are the implications for my dobermann’s health and life expectancy?

The treatment is fairly simple and inexpensive. The dog will be given synthetic thyroxine (called levothyroxine or L-thyroxine), either in tablet or liquid form, which it will need to take for life. The vet will check periodically to ensure that the dog’s thyroid levels remain normal and it does not become hyperthyroid. There is no reason why a hypothyroid dog should not have a normal lifespan and lead an active life. Buying ‘alternative’ types of thyroxine replacement (eg pig thyroid, often marketed under the brand name Armour) is not recommended as their proportions of T3 and T4 can be variable and anyway are unlikely to be in the correct proportions for dogs.

Advice for puppy purchasers

Given that the dobermann gene pool is fairly small, and that hypothyroidism is not generally fatal and is one of the more easily managed genetic diseases, rigorously attempting to exclude hypothyroidism, even if this were possible, might just cause other genetic defects to be magnified, or new ones to emerge. The prevalence of hypothyroidism in human women is thought to be as high as 9%, which hopefully puts this into proportion.

Advice for breeders

If a bloodline seems to be especially susceptible to hypothyroidism, it makes sense to look for suitable mates whose bloodlines do not seem to be susceptible, as evidenced by a recent negative TgAA test and/or by enquiring about antecedents, but only where this is consistent with other positive health indications. Given the relatively small effects on health and longevity compared to DCM and cancer, it is important not to put too much emphasis on hypothyroidism.

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