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
ALL RIGHTS RESERVED.