Studies of blood glucose, plasma insulin and free fatty acid levels during oral glucose loading in thyrotoxicosis
An abnormal glucose tolerance curve is a common accompaniment of thyrotoxicosis. The increased frequency of diabetes mellitus in hyperthyroidism and the adverse effect of hyperthyroidism on patients with diabetes mellitus are well established (Kreins et al., 1965).
The mechanism contributing to this impaired carbohydrate tolerance is unclear. Glucose tolerance is dependent upon the interaction of tissue sensitivity to insulin and the magnitude of insulin secretion. During the hyperthyroid state, tissue sensitivity to insulin has been variably reported to be increased (Elrick et al., 1961) or decreased (Doar et al., 1969).
The results of insulin secretion studies have also been quite varied; decreased (Renauld et al., 1971), normal (Hales and Hyams, 1964), or increased (Doar et al.,1969).
In thyrotoxicosis elevation of the fasting plasma free fatty acid(FFA) level and impairement of glucose tolerance are common (Rich et al., 1959, Marks et al.,1960). Hales and Hyams(1964) advanced the hypothesis that the reduced glucose tolerance in thyrotoxic patients was secondarily to an increased concentration of FFA in plasma.
The present investigations are carried out with the object of determining whether there are any relationship among the impairement of glucose tolerance, plasma insulin and plasma FFA concentrations. The blood glucose, plasma insulin and plasma FFA concentrations during 50 gm oral glucose loading were measured in 8 normal and 12 thyrotoxic subjects.
The result are as followings
1) By Wilkerson's criteria, eight of twelve thyrotoxic subjects (67 per cent) had abnormally high glucose level after oral glucose loading. The mean blood glucose level of the thyrotoxic patients after oral glucose loading was significantly elevated in comparing with that of the control subjects.
2) The Mean plasma insulin concentrations were not significantly different between two groups, but postprandial one hour value was significantly increased in the thyrotoxic group.
3) The fasting plasma FFA level of the thyrotoxic group was 1.8 times higher than that of the control group and after glucose loading plasma FFA concentration was decreased more rapidly in the thyrotoxic patients.
In view of the above experimental findings, the mechanism of abnormal carbohydrate metabolism in the patients with thyrotoxicosis may be interpreted as deficiency of insulin effectivity rather than that of insulin secretion.