Small Animal Dermatology, 3rd edition

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