protein–bound iodine

:  the amount of iodine expressed in micrograms per 100 milliliters of blood serum that is precipitated with serum proteins and that serves as a measure of the activity of the thyroid gland the protein-bound iodine in the normal human being ranges from 4 to 8 micrograms, in hypothyroidism falls below this range, and in hyperthyroidism rises above it