Page 1063 - Cote clinical veterinary advisor dogs and cats 4th

526 Hypothyroidism

○ Most accurate single-hormone test for follicles are commonly in a hairless, telogen ○ Standardize food administration (because
phase.
diagnosis of hypothyroidism • Most patients, even those without hypothy- it affects L-thyroxine absorption) and
VetBooks.ir canine TSH assay should be used. The test roidism, temporarily show some response • Monitoring clinical signs: expected evolution
timing of blood sample between visits.
• Basal serum TSH concentration: a validated
should not be evaluated alone but is best
to therapy with thyroid hormones (e.g.,
of response
used in conjunction with T 4 and/or FT 4 ED
therapy as a confirmatory test should be
commonly is seen within 1-2 weeks.
results. increased activity). Therefore, response to ○ An increase in alertness and activity
○ High TSH with low T 4 : sensitivity of avoided whenever possible. ○ Neurologic improvement may begin
63%-67%, specificity of 98%-100%, within the first month, but several months
accuracy of 82%-88% TREATMENT may be needed for full resolution; some
○ High TSH with low FT 4 ED: sensitivity neurologic manifestations do not resolve.
of 74%, specificity of 98%, accuracy of Treatment Overview ○ Dermatologic improvement often takes
86% The mainstay of treatment is oral thyroid 1-4 months.
○ Increased TSH with decreased T 4 hormone replacement. Therapy should be ○ Resolution of reproductive manifestations
or FT 4 ED is strongly supportive of designed to raise serum T 4 values to the normal may take several months.
hypothyroidism. range and eliminate clinical signs. Therapy is • If major clinical improvement is not seen in 3
○ A normal TSH in conjunction with lifelong. months despite normal serum T 4 concentra-
decreased T 4 or FT 4 ED does not rule out tions, a concurrent but unidentified disease
hypothyroidism, and if clinically relevant, Acute General Treatment should be considered.
advanced thyroid testing is recommended. • Hypothyroidism usually is a chronic condi-
• See relevant laboratory tests (pp. 1385 and tion that seldom requires acute therapy. PROGNOSIS & OUTCOME
1386). • Myxedema coma (rare): levothyroxine for
• Measurement of serum T3 concentra- injection 5 mcg/kg IV q 12h until oral • Primary hypothyroidism: long-term prog-
tion is not helpful for the diagnosis of administration possible nosis is excellent with adequate lifelong
hypothyroidism. therapy.
Chronic Treatment • Secondary hypothyroidism: long-term
Advanced or Confirmatory Testing Oral thyroid supplementation: levothyroxine prognosis is usually guarded because pituitary
If basal thyroid hormone concentrations do sodium (synthetic T 4 ): neoplasia is the most common underlying
not rule in/out hypothyroidism, additional • Initial dose (tablets) cause.
options exist. TSH stimulation tests and thyroid ○ In dogs, 0.01-0.02 mg/kg PO q 12h.
scintigraphy are considered gold standards. Maximum dose 0.8 mg/DOG PO q 12h. PEARLS & CONSIDERATIONS
• TSH stimulation test If twice-daily administration is a problem,
○ Several protocols exist; consult the a once-daily dosage of 0.02 mg/kg is suf- Comments
laboratory performing the thyroid assays ficient in many dogs. • The presence of nonthyroidal illness can make
for the desired protocol and reference ○ In cats, initial dosage is 0.075 mg/CAT q it difficult to obtain a definitive diagnosis of
ranges for baseline and poststimulation 12h. hypothyroidism. Sick euthyroid syndrome
T 4 concentrations. • A veterinary-licensed liquid formulation of describes the condition that occurs when
○ Recombinant human TSH aliquoted L-thyroxine is available in many countries nonthyroidal illness results in decreased
and frozen in small syringes is used (not the United States); dose in dogs is basal T 4 (and less commonly, FT 4 ED)
(50-150 mcg IV); evaluate serum T 4 0.02 mg/kg PO q 24h. concentrations. Measurement of multiple
concentrations in samples drawn before • With concurrent heart failure, kidney thyroid hormones and advanced testing
administration and 6-hour after admin- disease, liver disease, hypoadrenocorticism, may be necessary.
istration. In patients with suspected or diabetes mellitus, the initial dose should • Subnormal T 4 concentrations are not an
or confirmed concurrent nonthyroidal be decreased by 25%-50%, then slowly immediate indication for supplementation
systemic illness or receiving medication increased over the next 2-4 months. with levothyroxine, which may sometimes be
that may lower total T 4 concentrations, • A brand-name veterinary preparation should deleterious. The history and physical exam
the higher dosage should be used. be used because bioavailability of generic findings must be critically evaluated for
• Thyroid scintigraphy can differentiate forms can vary. features supportive of hypothyroidism and
between hypothyroidism and nonthyroidal • Adjust dose based on clinical response and signs of other illness that could be causing
illness but is limited by availability. serum T 4 concentrations (see Recommended sick euthyroid syndrome. In the latter case,
• Serum thyroglobulin antibodies: increased Monitoring, below). resolution/treatment of nonthyroidal illness
levels suggest the presence of lymphocytic • Absorption kinetics vary between brands, and returns T 4 concentrations to normal.
thyroiditis but occur in only ≈60% of serum T 4 concentrations should be reassessed • Most obese dogs are not hypothyroid.
hypothyroid dogs. Lymphocytic thyroiditis if the levothyroxine brand is changed. • Certain breeds (e.g., sighthounds, Shar-peis)
does not correlate with the presence of or have lower T 4 values than other breeds.
always lead to hypothyroidism. Possible Complications • Many drugs can decrease T 4 concentrations.
• Serum thyroid hormone antibodies: can Iatrogenic hyperthyroidism Phenobarbital and especially sulfonamides
interfere with testing and lead to spuriously can lead to increased TSH levels, making a
high T 4 results, depending on the T 4 assay Recommended Monitoring reliable diagnosis very difficult. Sulfonamides
used • A physical exam and serum thyroxine should be stopped 2 months before thyroid
• Histopathologic evaluation of skin biop- concentration (4-6 hours after pill) should evaluation. When a patient is receiving them,
sies: epidermal and follicular infundibular be evaluated at 4-6 weeks, then q 6 months advanced thyroid testing is recommended
hyperplasia is usually found. Vacuolated or after euthyroidism is achieved. (TSH stimulation or scintigraphy).
hypertrophied erector pili muscles, increased ○ Serum T 4 concentrations (4-6 hours after
dermal mucin, and a thickened dermis are pill) should be in the upper one-half or Prevention
consistent with hypothyroidism. If biopsy just above the normal range. In most cases, Breeding of hypothyroid dogs risks perpetuating
samples are taken from areas with alopecia, monitoring serum TSH concentrations this disease; the presence of hypothyroidism
signs of hair growth cycle arrest are seen, and does not offer any additional advantage. should be considered together with the status

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