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. Journal of the…+1

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%. Journal of Vete… 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. Journal of Vete…+1 A low TT4 with a normal TSH cannot distinguish primary hypothyroidism from nonthyroidal illness syndrome, which independently suppresses TT4. Journal of Vete…+1 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. Journal of Vete…

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. Journal of Vete…+1

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. Journal of the… 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. Journal of the… An alternative published starting dose of 20 µg/kg (0.020 mg/kg) PO q24h achieves clinical improvement or resolution in 90%–91% of dogs after 4 weeks, with this dose suitable for once-daily maintenance in 79%; 15% require 30 µg/kg/day and 6% require 10–15 µg/kg/day. Journal of Vete… At 0.020 mg/kg q24h, peak TT4 is reached within 5 hours (tmax <5 hours), mean peak TT4 concentration is 56 ± 11 nmol/L, and apparent terminal half-life is 11.8 hours. Journal of Vete…

Monitoring targets guide all dose adjustments. Post-pill TT4 measured at peak (4–6 hours after dosing) should be interpreted alongside clinical response. Activity and metabolic signs improve rapidly, often within the first week of treatment, while dermatologic signs may take several months to resolve. Journal of Vete… Signs of thyrotoxicosis — panting, hyperactivity, polyuria, and polydipsia — are reported more frequently at starting doses of 0.044 mg/kg/day than at 0.022 mg/kg/day, supporting the lower starting dose. Journal of the… At 10× the initial dose in euthyroid dogs, vomiting, diarrhea, excitation, rapid respiration, tachycardia, and bloody feces were observed. FDA DailyMed An…

Dogs requiring twice-daily dosing should receive a 30% total daily dose increase distributed across the two doses. Journal of Vete… Minor upward adjustments of 9%–16% are sufficient for most dogs that do not reach target on the standard starting dose. Journal of Vete…

Would you like guidance on how long to wait after discontinuing phenobarbital or glucocorticoids before thyroid testing can be reliably interpreted?