Clinical signs of canine hypothyroidism often manifest in middle age, with a mean age at diagnosis reported to be 6.8 yr.¹² Signs may be subtle and slowly progress over months to years. Dermatologic abnormalities occur frequently and often include truncal nonpruritic alopecia, “rat tail” appearance, poor coat quality, seborrhea, hyperpigmentation, and recurrent pyoderma.⁶ Other commonly reported clinical signs include lethargy, mental dullness, exercise intolerance, obesity or unexplained weight gain, and heat-seeking behaviors. Less commonly reported associations include facial nerve paralysis, vestibular disease, and polyneuropathy.¹³

Abnormalities on initial biochemical screening are nonspecific, with a fasted hypercholesterolemia, hypertriglyceridemia, and mild nonregenerative anemia most commonly found.¹⁴ Hypothyroid dogs are expected to have a total thyroxine concentration (TT4) below the laboratory reference range; a result in the upper half of the reference range generally excludes the condition. If clinical suspicion of hypothyroidism is high in a patient with a TT4 below or in the lower end of the reference interval, evaluation of free T4 (fT4) and TSH concentrations is warranted. Although equilibrium dialysis is preferred for fT4 testing (fT4ed), it is not essential, and assay availability should be confirmed with the laboratory. Definitive hypothyroidism diagnosis is characterized by TT4 and fT4 concentrations below the reference interval with a TSH concentration above the reference interval; however, ~20–40% of dogs with overt hypothyroidism will have TSH concentrations within the reference range.^15,16 Therefore, having two of the three hormone concentrations indicative of hypothyroidism is enough to support the diagnosis in a patient with compatible clinical or biochemical abnormalities. An isolated TT4 below the reference interval should not be the only criteria used to diagnose hypothyroidism. Practitioners must consider that reduction of TT4 concentrations can occur secondary to patient breed, advancing age, and administration of certain medications, such as prednisone, as well as during periods of illness or stress.^17,18 Severe illness can also suppress fT4 concentrations, further mimicking a hypothyroid state; therefore, thyroid testing should ideally be performed following clinical stabilization or disease recovery when possible, to maximize the diagnostic accuracy of results.¹⁹