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.