Abstract
Two genetically distinct forms of erythrocyte carbonic anhydrase, form B and form C, have been assayed by means of electrophoresis on cellulose acetate membranes in 92 euthyroid adults and in 49 patients suffering from thyroid disorders. A negative correlation was found between the concentration of erythrocyte carbonic anhydrase form B and plasma thyroxine. This finding is shown to be due to an effect of the thyroid hormone system on the synthesis of isoenzyme B in the red blood cell precursors. The use of the carbonic anhydrase B assay as a means of measuring the action of the thyroid hormone on the body cells is discussed.