What TSH and T4 thresholds should guide diagnosis and levothyroxine dosing in hypothyroid dogs?

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Primary hypothyroidism in dogs is confirmed by a low total thyroxine (TT4) combined with an elevated thyroid-stimulating hormone (TSH), and levothyroxine sodium is initiated at 0.022 mg/kg PO q24h (0.022 mg/kg/day total) as the FDA-approved starting dose. 313 Diagnostic thresholds require pairing TT4 and TSH, as neither test is sufficient alone. TT4 sensitivity is 89%–100% and specificity is 73%–82%; TSH sensitivity is 58%–87% and specificity is 82%–100%; free thyroxine (fT4) sensitivity is 80%–98% and specificity is 78%–94%. 4 Approximately 25%–38% of dogs with confirmed primary hypothyroidism have a TSH concentration within the reference range, meaning a normal TSH does not rule out the diagnosis. 810 A low TT4 with a normal TSH cannot distinguish primary hypothyroidism from nonthyroidal illness syndrome, which independently suppresses TT4. 1011 Diagnosis should only be pursued in dogs with clinical and clinicopathologic findings compatible with hypothyroidism, as pre-test probability directly determines the predictive value of thyroid function tests. 4 Several drugs significantly distort thyroid test results and must be accounted for before interpreting any panel. Phenobarbital decreases serum TT4 and increases serum TSH. Prednisolone decreases serum TT3, TT4, and fT4 in a dose- and duration-dependent manner but does not affect TSH. Sulfonamides can induce true hypothyroidism, reversible upon discontinuation. Glucocorticoids, aspirin, ketoprofen, carprofen, clomipramine, and sulfonamides all affect thyroid function test results. 48 The FDA-approved starting dose for Thyro-Tabs Canine is 0.022 mg/kg PO q24h, given as a single daily dose or divided q12h — both schedules produce equivalent therapeutic monitoring results and resolution of clinical signs. 3 The labeled maintenance dose range is 0.009–0.044 mg/kg/day, with 80% of dogs maintained between 0.018 and 0.026 mg/kg/day. 3 An alternative published starting dose of 20 µg/kg (0.020 mg/kg) PO q24h achieves clinic