Intra- and interindividual components of biological variation have been determined for total thyroxin (TT4), free thyroxin (FT4), total triiodothyronine (TT3), free triiodothyronine (FT3), and thyrotropin (TSH). Calculated analytical goals (CV, %) for the precision required for optimal patient care are: TT4 less than or equal to 2.5, FT4 less than or equal to 4.7, TT3 less than or equal to 5.2, FT3 less than or equal to 3.9, and TSH less than or equal to 8.1. The marked degree of individuality demonstrated for all hormones indicates that, if conventional population-based reference ranges are used uncritically, major changes in hormone concentration may not be correctly identified for some patients because observed values continue to lie within the reference range. At analyte concentrations approximating the mean values found in this study, and for analytical performance meeting the appropriate analytical goal, the differences required for consecutive results to be significantly different (p less than or equal to 0.5) have been calculated as: TT4, 14.7 nmol/L; FT4, 5.7 pmol/L; TT3, 0.6 nmol/L; FT3, 1.3 pmol/L, and TSH, 0.7 milli-int. unit/L.