Postoperative replacement doses of sodium-L-thyroxine in patients with thyroid carcinomas.

The serum concentration of thyroxine and triiodothyronine, measured by a radioimmunoassay technique, was followed during replacement therapy of l-thyroxine in 10 totally thyroidectomized patients in order to evaluate the clinical usefulness of the assay for the estimation of the optimal replacement dose. The results indicate that the optimal dose of l-thyroxine yields a serum concentration of l-thyroxine in the high toxic range, without causing thyrotoxic symptoms. The concentration of triiodothyronine is simultaneously on the borderline to the toxic range. It is thus necessary to follow the concentration of triiodothyronine during substitution in order to titrate the optimal replacement dose of l-thyroxine. The determination of thyroxine in serum is of secondary value for this purpose.