lntra-lndividualVariationof Thyroxin,Triiodothyronine, and Thyrotropinin Treated Hypothyroid Patients: Implicationsfor MonitoringReplacementTherapy

We measured total thyroxin (Tr4), free thyroxin(FT4, total triiodothyronine (TT3), free triiodothyronine (FT3), and thyrotropin (TSH) in serum sampled before and 1, 2, 4, 6, and 8 h after administration of prescribed doses of thyroxin to 12 patients with proven primary hypothyroidism. At 2, 4, and 6 h post-dose, the mean values for 114 and FT4 and also that at 8 h for FT4 significantly (P <0.05) exceeded the corresponding pre-dose values. No significant changes were found for 113, Fl3, or TSH. The mean intra-individual CV5 over the study period were TT4 4.9%, FT4 5.7%, 1138.7%, FT3 8.7%, and TSH 20.2%. Individualsubjectsshowed small but predictable changes in 114 and FT4. Changes in 113 and FT3 were greater but random.Fluctuationsin TSH were greatest, but in all subjectswith detectable concentrationsthe variations were of similar magnitude. We conclude that strict adherence to timing of specimen collection in relation to dosage is probablyunnecessary.