Small Animal Dermatology, 3rd edition

CHAPTER 28 HYPOTHYROIDISM 421

Hypothyroid: pre- and post-T4 results below normal range

Requires 8-week withdrawal period for dogs currently on T

4 supplementation

for accurate TSH stimulation testing

TSH stimulation testing can be run concurrently with corticotropin stimulation

testing or a dexamethasone suppression test with no compromise in accuracy

Recombinant human TSH can be used but is prohibitively expensive.

 Additional tests:
TRH stimulation test: response variable and smaller than with TSH; not practi-

cal; may help differentiate primary hypothyroidism from secondary or tertiary

hypothyroidism

Thyroid gland biopsy: definitive diagnostic test; impractical with potential com-

plications; pathologic findings in lymphocytic thyroiditis include infiltration of

lymphocytes, macrophages and plasma cells with eventual destruction of thyroid

parenchyma and replacement with fibrous connective tissue

Thyroid gland ultrasound: useful; euthyroid and sick-euthyroid dogs will have

normal thyroid gland size; decreased thyroid gland size in hypothyroid dogs

Therapeutic trial: unreliable for diagnosis of hypothyroidism based on dermato-

logic abnormalities; thyroid hormone supplementation produces similar initial

effects (e.g., hair growth, increased activity) in both euthyroid and hypothyroid

dogs; prompt measurement of post-pill T 4 levels may reveal significantly elevated

results in euthyroid dogs

Dermatohistopathology: often not specifically diagnostic for hypothyroidism;

more indicative of endocrinopathy; epidermal and follicular infundibular hyper-

plasia and hyperkeratosis (epidermal atrophy more commonly noted in other

endocrinopathies); predominance of telogen follicles; sebaceous gland hyper-

plasia; mucinosis/myxedema, vacuolated arrector pili muscle, frequent finding

of bacterial folliculitis.

THERAPEUTICS

General Considerations

 Diet: reduced fat as indicated by abnormalities in serum lipid measurements.

 Life-long treatment necessary.

 Response to therapy: within 7 days for neuropathy and severe systemic signs;

4–6 weeks for laboratory abnormalities and general systemic signs; 1–3 months for

dermatologic conditions (Figure 28.11).

 Thyrotoxicosis: seen as tachycardia, diarrhea, polyuria, polydypsia, polyphagia, pru-

ritus, and anxiousness or behavioral changes.

Drugs of Choice

 Synthetic L-thyroxine (levothyroxine sodium): supplement of choice.

 Bioavailability may vary between formulations and with generics.