Determinations of protein-bound iodine levels were carried out on serums obtained from 1,823 men in industry, 2,182 male commercial blood donors, 735 male physicians, and 1,015 women employees of a general hospital. The mean for the first group of men was 5.33 mcg. % with a standard deviation of ±1.19; the mean for the women was 5.67 mcg.% with a standard deviation of ±1.22. On the basis of interviews, physical examinations, and laboratory studies on smaller groups of subjects it was concluded that protein-bound iodine values below 4 mcg.% in men or 4.5 mcg.% in women are diagnostic of hypothyroidism. In these four populations, such evidence of hypothyroidism was found in 4.94%, 9.2%, and 4.2% among the men and 7.3% among the women.