Intra-individual variation of thyroxin, triiodothyronine, and thyrotropin in treated hypothyroid patients: implications for monitoring replacement therapy.

Abstract

Abstract We measured total thyroxin (TT4), 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 TT4 and FT4 and also that at 8 h for FT4 significantly (P less than 0.05) exceeded the corresponding pre-dose values. No significant changes were found for TT3, FT3, or TSH. The mean intra-individual CVs over the study period were TT4 4.9%, FT4 5.7%, TT3 8.7%, FT3 8.7%, and TSH 20.2%. Individual subjects showed small but predictable changes in TT4 and FT4. Changes in TT3 and FT3 were greater but random. Fluctuations in TSH were greatest, but in all subjects with detectable concentrations the variations were of similar magnitude. We conclude that strict adherence to timing of specimen collection in relation to dosage is probably unnecessary.