Cause
Canine hypothyroidism is caused by progressive destruction of the thyroid
glands by either lymphocytic thyroiditis or idiopathic atrophy. The underlying
cause of these pathologies is unknown but some dogs appear genetically predisposed.
It has been suggested that functional failure occurs once approximately
75 percent of the thyroid glands have been destroyed.
Signalment and Clinical Signs
Any breed can be affected: Clinical signs can be extensive but over 75 percent
of cases have both dermatological and metabolic abnormalities. Dermatological
signs include an "endocrine" alopecia often affecting the flanks,
thighs, neck and dorsum; hyperpigmentation, skin thickening, poor quality
hair coat, excess scurf and scale, seborrhoea and secondary pyoderma. Metabolic
signs include weight gain (it is a myth that all hypothyroid dogs are
fat but many will have recently put on some weight), exercise intolerance,
lethargy, mental dullness, excessive sleeping. Less common clinical manifestations
include peripheral neuropathies, corneal lipidosis, subnormal reproductive
performance (females), amongst many others.
Diagnostic Tests
Routine biochemistry usually demonstrates hypercholesterolaemia (often
marked) and haematology frequently shows a mild poorly regenerative
anaemia. The most important reason for performing these routine tests is
to help eliminate the presence of nonthyroidal illness that may subsequently
interfere with the results of thyroid function tests. Total T4 is invariably
subnormal in hypothyroidism and so a "normal" total T4 strongly
suggests euthyroidism. However, subnormal results are also very common in
euthyroid dogs and this test cannot be used in isolation. Thyrotropin (thyroid
stimulating hormone, cTSH) is increased in 80 percent hypothyroid dogs
due to loss of the negative feedback effect of thyroid hormones on the pituitary
gland. Combined total T4 and cTSH is an inexpensive reasonably reliable
screening profile for hypothyroidism. Free T4 is less affected by nonthyroidal
factors than total T4 and so low values although still not 100% confirmatory
for hypothyroidism are more suggestive of it than low total T4 values. Free
T4 is particularly useful in dogs with decreased total T4 and reference
range cTSH results since if due
to nonthyroidal illness (usually responsible for this combination of
results) then free T4 is generally in the reference range. Note: Free
T4 must be measured by equilibrium dialysis (the method used by Axiom).
Ensure that if you request free T4 from any laboratory that this is the
method used. Thyroglobulin antibodies (TgAb) are present in dogs
with lymphocytic thyroiditis:
the most common cause of hypothyroidism. The presence of TgAb are a very
specific test for thyroid pathology and in the presence of reduced thyroid
function and appropriate clinical signs strongly support hypothyroidism.
A negative TgAb result is of little diagnostic significance since
this is common in hypothyroid and euthyroid dogs.
Treatment
For guidance on therapy and therapeutic monitoring, see Axiom Endocrine
Fact Sheet 1.1. |