Immune Mediated Hypothyroidism
Immune Mediated Hypothyroidism is caused by the underactivity of the thyroid gland and its failure to produce adequate amounts of the thyroid hormone T4. There is a feedback mechanism with the pituitary gland which normally produces a hormone known as TSH or Thyroid Stimulating Hormone. If the level of T4 drops the pituitary produces more TSH to stimulate production. If there is too much T4 then the production of TSH is reduced.
Although hypothyroidism can be caused by any interference in this feedback system in dogs 95% of cases are due to the auto-destruction of the thyroid gland by anti thyroid antibodies, causing primary hypothyroidism. This is sometimes referred to as lymphocytic thyroiditis
Much more rarely a secondary form of hypothyroidism is due to the destruction of the pituitary gland tissues that produce TSH. Usually this is due to a tumour and defects of other pituitary hormones will occur.
With the auto-immune condition the tissues of the thyroid gland are infiltrated by lymphocytes, plasma cells and macrophages resulting in steady destruction of the follicles which produce the hormone T4
The condition most commonly develops in Beardies between the ages of 4 and 10. There does not seem to be a sex predilection but speyed bitches appear to have a higher risk compared to intact bitches.
The thyroid hormone is the controller of the energy systems and metabolism, if the dog is becoming hypothyroid it is like setting a thermostat too low. The dog may show signs of mental dullness, lethargy, intolerance of exercise and weight gain without an increase in appetite. Difficulty in maintaining body temperature may sometimes cause frank hypothermia in the more severe cases. The classic hypothyroid Beardie will seek out heat beside a fire or radiator, whereas the normal dog dislikes excess heat. Alterations in the skin and coat are very common. There may well be excessive shedding of the coat when being groomed, the hair may become sparse and coarser. The skin becoming dry and flaky. The tissues on the forehead and face may become puffy with thickend skin folds on the forehead and puffy upper eye lids. This has been described as a “tragic” facial expression. In the most extreme form of the condition if untreated it can slow the dog right down so it becomes comatose the so called myxoedemic coma
This condition tends to be over diagnosed without proper reference to a variety of blood tests available to help. Empirical treatment with throxine just because of a poor coat must be avoided. It is necessary to run a whole batch of blood tests often referred to as a “thyroid panel” These test should include the serum total T4, free T4, TSH, and thyroid antibody tests. A provocative TSH stimulation test may be helpful but is very expensive as TSH is in short supply. Most hypothyroid dogs will also have a raised serum cholesterol.
A good single test to start with is the total T4. A dog with a normal level is 90% likely to be normal with no evidence of hypothyroidism. However a low T4 by itself is not diagnostic, it may be the dog is hypothyroid but could also be normal or have a non thyroid cause of the illness and have what is known as the sick euthyroid syndrome. Euthyroid simply means normal thyroid. A high level of T4 would indicate over activity or hyperthyroidism ( Thyrotoxicosis) this is very rare in Beardies, unless they are being over-treated with Soloxine.
Interpretation of thyroid function tests is quite complex especially when T3 levels are measured. The TSH level which would be expected to rise may well stay normal for some time in up to 40% of confirmed hypothyroidism cases, making TSH of little use alone in making the diagnosis. In humans a high TSH is highly suggestive of hypothyroidism and is the initial screening test of choice.
Treatment is aimed at adjusting the daily dose of thyroxine usually given as the commercially available soloxin to maintain normal Total T4 and TSH levels. Regular blood tests initially and then once stable, every year to ensure the levels remain constant with dose adjustment if necessary