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Salivary Thyroxine as an Estimate of Free Thyroxine: Concise Communication

Veterans Administration Medical Center and University of Minnesota, Minneapolis, Minnesota

Correspondence: For reprints contact: Dr. Shafer, Nuclear Medicine Service (115), VA Medical Center, 54th Street & 48th Avenue South, Minneapolis, MN 55417.

ABSTRACT

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.