Increased serum concentration of free L-triiodothyronine in patients treated with L-thyroxine

This study demonstrates that in spite of measured normal concentrations of carrier proteins one cannot deduce in all cases a normal fT3 from a normal level of TT3 when l-thyroxine given for diagnostic or therapeutic purposes is present in excess. The displacement of l-triiodothyronine from its binding sites is shown in 35 patients with non-toxic goitre who received an oral dose of 200 micrograms l-thyroxine/die for two weeks. Apart from a significant increase of TT4 (from 7.85 to 14.21 micrograms/dl equal + 81%) and of fT4 (from 1.58 to 3.7 ng/dl equal + 134%) there is only a slight increase in TT3 from 148 to 158 ng/dl (equal + 10%) after 14 days of treatment. By contrast fT3 rises clearly from 4.97 to 8.07 pg/ml equal + 63% (normal range: 2.8-5.6 pg/ml). Compared with the increase of TT3 (+ 10%) the free T3 rises by a factor of 6.3 (63%/10%). On account of higher affinity of l-thyroxine to binding proteins the free T4 is influenced to a lesser degree. Compared with the increase of TT4 (+ 81%) free T4 rises by a factor of 1.6 (134%/81%). It is supposed that the serum concentration of free T3 can be increased despite a normal concentration of TT3 when l-thyroxine is present in excess. Therefore, for laboratory work fT3 should be assigned a higher validity than TT3 when patients are treated with comparatively high doses of l-thyroxine.