Iodine and Thyroid Function: Physiological Significance of the Plasma Inorganic Iodine1

The plasma inorganic iodine (PII) has been estimated by an isotope dilution technique in 34 males and 37 females without evidence of thyroid disease. Significant findings include: 1) The mean PII is the same in both sexes (0.18 μg/100 ml); 2) females, but not males, show a significant negative correlation between the PII and the thyroidal radioiodine clearance; 3) there is a positive correlation between PII and absolute iodine uptake (AIU) in both sexes, suggesting that, the higher the PII, the more iodide is trapped by the thyroid per unit time; 4) there is a significant positive correlation of PII with age in females but not males; 5) females, but not males, show a negative correlation between PII and renal iodide clearance. It is concluded that there are differences in iodine metabolism between males and females, and they might contribute to the greater incidence of simple goiter in females.