The individual reference ranges for test results were narrow, compared with group reference
ranges used to develop laboratory reference
ranges. Accordingly, a test result within laboratory reference limits is not necessarily normal for an individual."
Andersen S, Pedersen KM, Bruun NH, Laurberg P. Narrow individual variations in serum T(4) and T(3) in normal subjects: a clue to the understanding of subclinical thyroid disease. J Clin Endocrinol Metab. 2002 Mar;87(3):1068-72.