Background: Biological variation of an analyte is an important determinant of its usefulness. Intraindividual variation in serum thyroid hormones, parathyroid hormone (PTH) and insulin-like growth factor-1 (IGF-1) in healthy subjects was determined in this study. Methods: Blood samples were taken at weekly intervals for six weeks from 10 healthy subjects and analysed for serum thyroid-stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), PTH and IGF-1. Results: Intraindividual variations for serum FT4 and FT3 were ,5.0%, IGF-1 9.4% and TSH and PTH 25.1% and 25.9%, respectively. Index of individuality was lowest for FT4 (0.42) and FT3 (0.38), and highest for PTH (1.09). Conclusion: Intraindividual variation for FT4, FT3 and IGF-1 are low.

Intra-individual variations for serum creatinine and cystatin C were similar in healthy subjects.