Radioimmunoassay of thyroxine in saliva

A simple radioimmunoassay (RIA) for thyroxine (T4) in saliva has been described. Fifty euthyroid control subjects, 14 euthyroid pregnant women, 23 thyreotoxic and 10 hypothyroid patients were examined. Serum T3, T4, thyroxine binding globulin (TBG) and TSH were measured simultaneously. The mean level of T4 in saliva in controls was 1.10 +/- 0.07 nmol/l. There was a good correlation between the saliva and serum T4 concentrations (r = 0.74) and between saliva T4 values and the T4/TBG ratio (r = 0.83). The saliva T4 levels, like serum free T4, were not dependent on fluctuations of serum TBG concentrations. In euthyroid pregnant women, saliva T4 levels were within the normal range while the serum T4 and TBG were increased. There was a good agreement of saliva T4 values with the functional state of the thyroid. Thus, the RIA of saliva T4 could replace the laborious determination of serum free T4. It can especially be useful in instances with abnormal values of TBG, as it is in pregnancy, in congenital deficiency of serum TBG or in subjects with hereditary elevated TBG levels.