Radioimmunoassay monitoring of thyroid hormone concentrations in dogs on thyroid replacement therapy: 2,674 cases (1985-1987)

Summary Serum iodothyronine concentrations from 4,064 samples submitted for monitoring of thyroid replacement therapy were evaluated in a retrospective study. After exclusion of samples because of the presence of 3,5,3’ triiodothyronine (T 3 ) autoantibodies, insufficient numbers of dogs on some commercial preparations or medication with corticosteroids or synthetic T 3 preparations, data from 2,674 dogs remained. Data were analyzed by using information on dose, time after dosing, commercial product, and once-a-day or twice-a-day dosing regimens. Serum total thyroxine (T 4 ) and total T 3 and estimates of free T 4 and free T 3 were significantly high in serum from dogs given higher doses of synthetic L-thyroxine orally. Doubling the oral dosage did not double the serum iodothyronine concentrations, perhaps because of poor absorption or more rapid catabolism of the hormones at higher L-thyroxine doses. Wide variation in the therapeutic hormone concentrations was found. Some dogs given low dosages of L-thyroxine had normal iodothyronine concentrations whereas some others given higher dosages had low normal to low concentrations. Monitoring the serum concentrations is an objective way to ensure adequate concentrations for successful therapy. When a therapeutic trial is used as a diagnostic procedure, one should not rule out hypothyroidism unless a therapeutic monitoring sample has indicated that replacement dose and absorption of the exogenous iodothyronine has been adequate. Thyroid hormone concentrations peaked at 4 to 6 hours after oral administration of L-thyroxine for once-a-day and twice-a-day dosage regimens. Higher concentrations were achieved with once-a-day than with twice-a-day regimens at the same total daily dose. Because 2 of the lower peaks occurred per day with twice-a-day dosing, total target tissue exposure to the iodothyronines appeared to be similar to the once-a-day exposure. In general, dogs receiving the veterinary preparation of sodium levothyroxine had higher serum concentrations of iodothyronines than did dogs receiving a commonly used human preparation or generic preparations.