Folic acid clearances and basal serum folate levels in patients with thyroid disease.

The plasma clearance of intravenously administered folic acid was abnormally rapid in all of five patients studied with untreated hyperthyroidism; in three, the basal serum L. casei folate level was subnormal. Four of these patients and another patient were studied at various intervals after partial or complete correction of the hyperthyroid state. Partial correction resulted in a continued fast clearance in one patient, a return to borderline-normal values in the second, and a normal clearance in the third. Complete euthyroidism was achieved in two patients; their clearance values had returned to borderline-normal at this time. In two patients studied after partial correction, the serum L. casei folate levels had become normal whereas the clearance values remained abnormally rapid. These findings suggest that hyperthyroidism in man is associated with depletion of folate stores and subclinical deficiency of this vitamin. This is attributed to an increased demand for folic acid in the hypermetabolic state.