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Thyroid glands lie on the upper part of the trachea. There function is to control the metabolism and because of this they affect every organ in the body with the hormons thyroxine (T4) and triiodothyroxine (T3). The production of T4 and T3 is conrolled by the pituitary gland. Pituitary gland is controlled by the hypothalamus via thyrotropin releasing hormone (TRH). TRH stimulates the pituitary gland to secrate thyroid-stimulating hormone (TSH) if the circulating T4 and T3 levels are insufficient. TSH then stimulates thyroid glands to raise the production of T4 and T3.
The opposite happens when thyroid hormone levels are sufficient. TRH and TSH levels drop and the T4 and T3 production decreases. This is a mechanism called feedback regulation and it is very typical for homeostasis of many hormones.

Hypothyroidism has two main ethiology. The other is lymphocytic thyroiditis, an autoimmune disorder. This is a condition where the immune system attacks the tissues of the thyroid gland distroying the cells gradually. The other is an idiopathic thyroid degeneration and atrophy. Hypothyroidism is a quite common disease affecting many breeds. There is a genetic predisposition to autoimmune thyroiditis although the mode of inheritance has not yet been defined and several enviromental factors can trigger the onset of the disease. The disease is slowly progressive, typically occuring in older dogs anyway not until 2 years of age.

Signs: There are no specific clinical signs in hypothyroidism. The signs are depending on the organ it affects and are various. Hypothryoid dogs can have any of these signs or a combination of signs. Decreased stimulation leads to decreased activity levels and increased body weight, obesity, lethargy or weakness. Skin and haicoat changes are typical. Hyperpigmentation, poor quality coat, excessive scaling and loss of hair in flank and back area is often seen. As immune system is also depressed there is an increased susceptability to infection also affecting the skin causing infected or itching skin. Heart rate may be slow. The dog may have intolerance to cold and muscle weakness. There are reports of infertility in males and failure of females to cycle although this is controversial. If left untreated the signs will slowly progress.

The signs of hypothyroidism are not specific, diseases like other endocrine imbalances (hyperadrenocorticism, hyperestrogenism, hyperandrogenism), congenital diseases (follicular dysplasia, colour dilution alopecia, seasonal flank alopecia) and various other problems can cause similar signs seen in hypothyroidism.

Many laboratory measurements are possible to test. T4 level alone is not reliable as T4 fluctuates throughout the day and there are two distinct fractions of T4 in the blood - free T4 and T4 bound to protein. The bound form is not possible to measure. T4 level can be determined by a blood test.
TSH level is a good concominant factor to be measured. It is not reliable to measure alone but in a connection with free T4 measurement it increases the reliability of the test near (but still not) to perfection. Hypothyroid dogs have both a high TSH level and a low free-T4 level.

There is also an euthyroid sick syndrome where the thyroid function is normal but the decresed thyroid hormone levels are due to some other concurrent medical illness, as part of a metabolic response to this nonthyroidal illness (diabetes, hyperadrenocorticism etc.). This can sometime lead to misdiagnoses and false-positive individuals can be found.

The treatment is generally easy. Levothyroxine is a drug of choice and is given orally twice daily. It will effectly correct the decresed production and thus the signs of hypothyroidism. Hormone supplementation is relatively inexpensive and side effects are uncommon. Oversupplementation can cause hyperthyroidism signs like hypeactivity, diarrhea, weight loss. The dose should be administered according the signs and blood levels of T4 and TSH should be rechecked periodically. The medication in hypothyroidism is life long but rewarding and most hypothyroid dogs can live normal and healthy life.

© Mervi Ihantola