The recommended starting dosage of levothyroxine sodium is 10 μg/kg bodyweight orally every 12 hours. Because of variability in absorption and metabolism, the dosage may require alterations before a complete clinical response is observed. The initial dosage and frequency of administration are merely a starting point. Therapy has to be highly individualised and tailored to the requirements of the individual dog.

To adequately monitor therapy, trough values (just prior to treatment) and peak values (approximately three hours after dosing) of plasma T4 can be measured. In adequately dosed dogs peak plasma concentrations of T4 should be in the high-normal range (approximately 30-47 ηmol/l) and trough values should be above 19 ηmol/l. If T4 levels are outside this range the levothyroxine sodium dose can be adjusted in 50 – 200 μg increments, until the patient is clinically euthyroid and serum T4 is within the reference range. Plasma T4 levels can be retested two weeks after a dosage change, but clinical improvement is equally an important factor in determining individual dosage and this will take four to eight weeks. When the optimum replacement dose has been attained, clinical and biochemical monitoring may be performed every 6-12 months.