Download The Normal Thyroid Gland

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The Normal Thyroid Gland
The thyroid gland produces hormones commonly called T3 and T4. These hormones are responsible for
regulating many biological functions of the body. They regulate such things as oxygen consumption, bone
formation and resorption, metabolism of body fat, protein production and the production of red blood cells –
just to name a few.
The amount of hormone released into the blood stream is regulated by what is called a negative feedback
system. You can refer to the diagram below to understand this feedback system.
TRH
Cells
Pituitary
Gland
TSH
T4
Thyroid Gland
TRH (thyrotropin releasing hormone) is released by the part of the brain called the hypothalamus on a
constant basis to stimulate the pituitary gland (tonic stimulation). The pituitary releases Thyroid Stimulating
Hormone (TSH). TSH tells the thyroid gland to produce and release T3 and T4. When sufficient levels of
T3/T4 are in the blood, the pituitary reduces the production of TSH and so the level of T3/T4 production is
reduced. In this way, the body regulates how much of these hormones are available to be used by the cells.
Hypothyroid Disease
Hypothyroid disease is a condition that occurs because of an inadequate level of thyroid hormone in the
body. This is usually because of a lack of hormone production due to either Lymphocytic Thyroiditis
(thought to be an immune system disorder) or Idiopathic Thyroid Atrophy (destruction of the gland due to an
unknown cause), although there are other less common causes. It is one of the most, if not the most,
common hormonal disease of dogs. It is rare in cats.
Signs and Symptoms of Hyperthyroid Disease
The signs of hypothyroid disease are related the low level of hormone. This leads to a decrease in the
overall body metabolism. Therefore, dogs with the disease often have one or more of the following
symptoms:
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Weight gain
Lethargy
Hair loss – often on both sides of the body
Poor hair growth following clipping
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Dry, dull coat
Increased Thirst
Increased Urination Frequency and Volume
Increased pigmentation of the skin
Skin infections
Diagnosis
If your pet has symptoms that may be consistent with hypothyroid disease, a T4 blood test performed in the
office is a very good screening test for your pet. If this test shows normal levels of T4, your pet likely does
not have a thyroid problem, and other tests will be suggested to determine the cause of your pets’
symptoms.
However, if your pet does have a lower than normal level of T4 consistent with hypothyroid disease, there
will be some further tests recommended to definitively diagnose your pets’ condition.
The additional tests are necessary because there are other disease states that can artificially lower the T4
level. If that is the case, these additional tests will help determine if that is occurring in your pet.
In most cases, it is a savings to you in time and money, as well as easier on your pet, to perform the simple
T4 screening test in the office. In that way, the additional thyroid testing is only indicated if the screening
test is positive.
If your pet has symptoms consistent with hypothyroid disease, it will also be important to perform a
complete blood count and full chemistry panel along with the T4 test. There are many other diseases that
have symptoms that are very similar to those of hyperthyroid disease. A full blood panel may reveal those
and help your doctor interpret all of the physical examination findings. In addition, hypothyroid disease can
often cause other problems, such as anemia, that are important to understand and may need to be treated
to return your pet to good health.
Treatment Options
If hypothyroid disease is confirmed in your pet, a daily medication will be prescribed, which will be given for
the rest of your pets’ life. This medication should return your pet to normal health, and if given as directed,
should prevent the symptoms from returning.
It is important to perform a medication titration during the initial phases of treatment. This means that you
will need to bring your pet in for T4 testing every few weeks until the proper dose of medication is
determined.
Post-treatment Care
The T4 level of your pet will need to be monitored on a regular basis. This is because the medication
requirements over time can change. Your pet may lose weight, or its’ metabolic requirements may change.
Whatever the cause, medication dosing often needs adjustment. We will discuss the recommended
frequency of testing for your pet, but in general this will occur at least once yearly.
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