There
are three categories of hypothyroidism, however the majority
of cases are due to the category labeled primary hypothyroidism.
Primary hypothyroidism can be divided into Iymphocytic hypothyroidism,
which is present most commonly, idiopathic atrophy and neoplastic
destruction. Lymphocytic hypothyroidism is due to a problem
with the immune system which leads to the destruction of
the thyroid gland. Idiopathic atrophy is a term used to describe
the condition where there is an unknown cause of fat replacing
functioning thyroid tissue. Neoplastic destruction is due
to an expanding tumor of the thyroid gland. Clinical signs
occur when more than 75% of the gland is destroyed.
Many
body systems can be affected with resultant lethargy, mental
dullness, exercise intolerance, weight gain, cold intolerance,
infertility, weak or still-born pups, lack of labido, testicular
atrophy, weakness, stiffness, muscle wasting, scaly skin,
dull coat, hair loss, 'Rat tail' appearance to the tail,
puppy type coat in an adult, skin infection, slow heart rate,
arrhythmias to the heart, ocular abnormalities, constipation,
diarrhea and vomiting. Lethargy, weight gain and symptoms
involving the coat and skin seem to be among the most common.
The most
common test of thyroid function is a blood test measuring
total T4, a hormone of the thyroid gland. Total T4 is T4
hormone bound and unbound to protein in the blood. Variation
in this value is possible due to certain medications such
as cortisone, age, presence of other illness, obesity and
breed type. When there is question about the validity of
a low total T4, other nonthyroidal illness would be explored
as well as a blood test to determine the levels of Free T4
(T4 unbound to protein) and Canine Thyroid Stimulating Hormone.
The initial
treatment of choice is administration of synthetic T4 (sodium
levothyroxine) which comes in a tablet form. Follow-up blood
serum levels would be assessed 4 to 6 weeks after initiating
supplementation and the dosage would be adjusted accordingly.
Clinical improvement for non-dermatologic symptoms may be
seen as early as I to 2 weeks, while dermatologic conditions
may take up to 4 weeks to improve after initiation |
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