Components of biological variation, including seasonality, in blood concentrations of TSH, TT3, FT4, PRL, cortisol and testosterone in healthy volunteers.

Objective: There are few detailed studies of annual or seasonal variations in hormone concentrations in man. This study examines the components of biological variation, including seasonality, in plasma TSH, total T3 (TT3), free T4 (FT4), PRL, cortisol and testosterone in healthy volunteers.

Design: Monthly blood samplings for the assay of the above hormones were collected during one calendar year.