Excessive or deficient iodine intake results in a variety of human thyroid dysfunction and disorder. The main part in the etiology of these disorders relates to the concentrations of intrathyroidal iodine which is important to regulate thyroid function and state. Instrumental neutron activation and X-ray fluorescent analyses were used to study the age dynamics of intrathyroidal iodine. Concentration and the total content of iodine in the thyroid were estimated in necropsy samples of 70 men and 20 women aged 2-87. Both thyroid lobes were weighed, lyophilised and homogenised. Iodine was analysed in approximately 50-mg samples. The mean intrathyroidal iodine concentration (mean +/- S.E.) of a normal subject aged 26-65 averaged 345 +/- 21 micrograms g-1 dry tissue in non-endemic goitre region with no obligatory salt iodination. Maximum iodine concentration was found to be 494 +/- 65 micrograms g-1 (P < 0.05) for the age of 16-25. A statistically reliable second increase of iodine was shown for the elderly which was 668 +/- 60 micrograms g-1 (P < 0.001) for the age over 65. Human intact thyroid weight for the age over 15 does not change and averages 14.2 +/- 0.4 g. Both left and right lobes of intact thyroid do not differ in weight, iodine concentration and the total content. An inverse correlation was found between thyroid weight and intrathyroidal iodine concentration (-0.32, P < 0.01). The range of intrathyroidal iodine parameters is wide enough to provide a possible explanation for the particular sensitivity of some population to both excessive and deficient iodine intake. Similarity of intrathyroidal iodine age dynamics and incidence of new cases of thyroid carcinoma were to confirm the hypothesis of intrathyroidal iodine importance in cancer etiologies suggested before.