If a cat treated with antithyroid medication has a serum TT4 concentration below the laboratory reference interval, the dose may need to be reduced, particularly if the cat is azotemic or if the TT4 is below the limit of detection. If the TT4 is above the limit of detection but below the laboratory reference interval and the cat is nonazotemic, continued monitoring may be appropriate if the cat is clinically well. Measurement of thyroid-stimulating hormone concentrations is also helpful for distinguishing cats with low serum TT4 concentrations secondary to nonthyroidal illness (including CKD) and iatrogenic hypothyroidism. When dose adjustments are required, the change should be in the smallest increments possible and the serum TT4 concentration retested every 3 to 4 weeks until the TT4 is within the lower half of the reference interval. Once euthyroidism is restored, renal parameters should be rechecked.
In cats treated with radioiodine or by surgical thyroidectomy that subsequently develop a low serum TT4 concentration, monitoring for up to 6 months may be prudent. If serum TT4 concentrations remain below the laboratory reference interval for more than 6 months, thyroxine supplementation should be instituted. In many cases, nonablated thyroid tissue restores normal thyroid function following definitive treatment of hyperthyroidism by radioiodine or surgery.
CKD = chronic kidney disease, GFR = glomerular filtration rate, TT4 = total thyroxine