[en] The effect of age and sex on thyroid functions was studied in 792 subjects who were proved by examination to be free from disease. This group comprised 436 males and 356 females and represented the different socioeconomic strata of the Egyptian population. Their ages ranged from 1 to 75 years. The thyroid function tests studied were performed by the solid phase single coated tubes radioimmunoassay technique. Results of the present study showed that serum total T3 (tT3), free T3 (fT3), and total T4 (tT4) were highest in subjects in the first decade of life, with higher levels in females than males. Thereafter, they dropped and remained around a plateau until the age of 40 years, then dropped again. Serum free T4 (fT4) behaved differently. In males, it was more or less the same, irrespective of age. In females, the highest values were seen during the first decade, and were higher than in males. Then fT4 dropped progressively to reach the levels recorded in males of the same age, where-after they became lower in females than in males above the age of 30 years. In contrast, estimation of T3 uptake (T3U), thyroid stimulating hormone (TSH), and thyroxine binding globulin (TBG) revealed that these parameters were not affected by either age or sex. The present results revealed the presence of three standards ranges of results, corresponding to the age groups 1-10, 11-40 and 41-75 years, for each of the two sexes separately. (author). 14 refs, 6 tabs