A comparison was made of serumhistaminase activity and calcitonin levels in forty-two patients with localised and metastatic medullary carcinoma of the thyroid. Thirty-three patients were members of families in which the disease was inherited. Half of the patients were found to have abnormally high serum-histaminase activity when compared with sixty-two controls. Twenty-six family members who did not have the disease had serum-histaminase activities within the normal range. Of the patients with metastatic tumour, 70% had high enzyme activity, with the highest activity in patients with pulmonary metastases. Serum-histaminase activity fell after removal of neck tumour in fourteen out of twenty patients, but did not fall to normal levels in six patients, four of whom had evidence of metastatic tumour. Basal serum-calcitonin levels were measured in thirty-one patients with medullary thyroid carcinoma and were above normal in twenty-nine. Calcitonin fell to low or undetectable levels in all but three of the patients who had neck tumour removed. These results suggest that measurements of serum-histaminase activity may be useful in the search for metastases and the detection of residual tumour after surgery. However, measurement of basal serum-calcitonin levels seems to be a more reliable test for early detection of localised medullary thyroid tumour.
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