RESUMEN
To establish reference intervals for Free Triiodothyronine [FT[3]] and Free Thyroxine [FT[4]] in euthyroid subjects and to assess the effect of age and gender on FT[3] and FT[4]. Cross-sectional, analytical study. Institute of Nuclear Medicine and Oncology [INMOL], Lahore, from January 2009 to April 2011. FT[3] and FT[4] were measured in 852 euthyroid patients. Those with previous thyroid surgery, on thyroidrelated medication and severe non-thyroid illness were excluded. There were 215 males and 637 females with mean age of 46 years. The reference intervals, revealed by this study, for FT[3] and FT[4] were 2.80 - 5.39 pmol/L and 11.9 - 22.2 pmol/L respectively. The mean difference between gender groups and age groups was found statistically significant for FT[3] [gender: p < 0.001; age: p < 0.001] but statistically insignificant for FT[4] [gender: p=0.147; age: p=0.201]. There was no effect of gender and age on FT[3] and FT[4]. The age with serum FT[3] and FT[4] significantly negatively correlated but this correlation was stronger for FT[3] as compared to FT[4] levels. Reference intervals of FT[3] and FT[4] for male [2.99 - 5.63 pmol/L, 12.3 - 22.6 pmol/L] and female [2.77 - 5.29 pmol/L, 11.7 - 22.1 pmol/L] respectively are suggested