420 DISEASES/DISORDERS
Sick euthryoid syndrome: low serum T
4 due to nonthyroidal illness
Specific diseases known to decrease baseline T
4 : renal failure, hepatic failure, sys-
temic and cutaneous infection, diabetes mellitus, hyperadrenocortisolism, and
hypoadrenocorticism
Specific medications known to decrease baseline T
4 : glucocorticoids, sul-
fonamides, nonsteroidal antiinflammatory drugs, phenobarbital, and tricyclic
antidepressants
In-house (ELISA) tests may be less reliable than radioimmunoassay tests.
fT 4 :
Measures active hormone
Recommend measure by equilibrium dialysis to remove effect of antithyro-
globulin antibodies
Less affected by concurrent diseases (with the exception of hyperadrenocorti-
solism)
Not recommended as a stand-alone test; many experts prefer measurement of
both total T 4 and fT 4 as the protocol for diagnosis of hypothyroidism.
Total T 3 /fT 3 /rT 3 :
Total T
3 levels fluctuate widely; measurement not an accurate indicator of thy-
roid gland status
Hypothyroid dogs often have normal T
3
No elevation in reverse T
3 levels in canine sick-euthyroid conditions as seen in
human patients
Autoantibodies to T
3 will affect results.
TSH:
High specificity, low sensitivity; useful as a confirmatory test, not as a stand-alone
test for diagnosis of hypothyroidism
Hypothyroid dogs may have normal TSH results; normal dogs may have elevated
TSH results
Elevated TSH in conjunction with low T
4 consistent with diagnosis of hypothy-
roidism
Further evaluation for thyroiditis or repeat measurement of values in 1–3 months
warranted for incongruent results.
Antithyroglobulin antibodies:
Found in more than 50% of hypothyroid dogs
Can occur in euthyroid dogs (25–35%)
May indicate developing thyroiditis in dogs with normal thyroid hormone levels;
may interfere with tests to falsely elevate total T 4
May be useful in dogs with equivocal thyroid hormone levels
May interfere with hormone assays resulting most often in increased test results.
TSH stimulation test:
Determines thyroid function
Baseline and post administration of TSH measurement of T
4
Euthyroid: post-T
4 result within or above high normal range for T 4
Nonthyroidal illness (sick-euthyroid): blunted response, but post-T
4 result
within normal range for T 4