Salivary thyroxine as an estimate of free thyroxine: concise communication.

To test the hypothesis that the levels of salivary thyroxine (T4) reflect those of circulating free T4, we developed a radioimmunoassay (RIA) sensitive to low levels of T4. Concurrent saliva and serum samples were obtained from 32 euthyroid volunteers, ages 19-64. Salivary and serum T4 and cortisol levels were measured by RIA. Salivary albumin was measured by nephelometry. Salivary T4 levels were higher than predicted, 4.2-35 ng/dl (normal range 0.6-2.0). No correlation was found between salivary T4 and serum levels of free T4 and total T4 but there was a significant correlation between salivary T4 and albumin (r = 0.82). Salivary cortisol levels agreed with reported results and showed no correlation with salivary albumin. We conclude that salivary levels of drugs and hormones may be strongly affected by protein binding, and caution must be exercised in using salivary levels as an estimate of circulating free levels.