The variation in serum triiodothyronine (T3) achieved during suppression testing in thyrotoxicosis

SUMMARY Regular measurement of thyroidal radioiodine uptake has been widely used as a means of monitoring continued extrapituitary stimulation of the thyroid during the treatment of thyrotoxicosis with carbimazole and triiodothyronine (T3). However, it is unclear to what extent the serum T3 level may vary at the time of testing, nor what effect this might have on the uptake of radioiodine. Two studies have been undertaken. In the first, serum T3 levels in twenty-four thyrotoxic patients were measured at intervals during an 18-month course of carbimazole combined with T3, 20 μg qid. Considerable variations in the highest and lowest levels of serum T3 were found both between and within individuals. The second study was on twenty-three thyrotoxic patients thought to be entering remission because the iodine uptake after 5 months of drug treatment had fallen to less than 50% of the pretreatment value (suppressors). The changes in uptake of radioiodine after 5 and then 6 months of treatment were compared in seven patients, who received carbimazole and T3 throughout, with the corresponding changes in the remaining sixteen patients, whose T3 alone was withdrawn prior to the uptake test at month 6. The mean degree of suppression remained unchanged by month 6 in the first group. In the second group, however, the mean uptake rose significantly, and nine of the sixteen patients would have been classified as non-suppressors at the sixth month (i.e. uptake greater than 50% the pretreatment value) had their failure to maintain high serum levels of T3 gone undetected. The first study indicated this could well happen in routine circumstances, and it is suggested that the reliability of suppression tests be checked with simultaneous measurement of serum T3.