A COMPARISON OF SERUM PBI AND THYROXINE IODINE LEVELS

ABSTRACT Serum PBI and thyroxine iodine (T4I) levels have been compared in a large series of patients. A mean difference of 0.35 μg/100 ml was found in euthyroid subjects, but a much greater difference obtained when both values were high, whether the elevation resulted from hyperthyroidism, pregnancy or the administration of thyroxine or oestrogen. This discrepancy was greater in a group of patients with diffuse goitre than in those with nodular glands. A circulating non-thyroxine iodinated compound is thought to account for this discrepancy, the origins of which could lie in the thyroid gland, although peripheral deiodination of thyroxine seems more likely.