Stimulation of thyroid adenylate cyclase activity by sera from patients with non-thyroidal illness

<jats:p>Abstract. We have previously shown that sera from many hypothyroid patients stimulated adenylate cyclase activity as measured by serum bioactive TSH concentrations produced by FRTL-5 cell line. This TSH-stimulating activity did not correlate with serum immunoreactive TSH. IgG fractions of these sera did not stimulate FRTL-5 cells. The present study was, therefore, undertaken to investigate the thyroid stimulating activity of sera from patients with non-thyroidal illness. Studies were performed in 36 patients with various non-thyroidal illness. In these patients, serum concentrations of T<jats:sub>4</jats:sub>, free thyroxine, T<jats:sub>3</jats:sub>, and TSH were determined. In addition, sera were incubated with FRTL-5 cells or porcine thyroid cells in primary culture in the presence of 0.4 m<jats:sc>m</jats:sc> MIBX, and medium cAMP concentrations were determined by radioimmunoassay. Sera obtained from some patients with various non-thyroidal illness increased cAMP concentrations in culture media of FRTL-5 cells as well as that of porcine thyroid cells. The thyroid stimulating effects of sera were not disease specific and significantly correlated inversely with serum T<jats:sub>3</jats:sub> and T<jats:sub>4</jats:sub> concentrations. Serum TSH concentrations in these patients were within the normal range even by the newly developed ultrasensitive assay. Although the nature of substance(s) present in sera of patients with low T<jats:sub>3</jats:sub> syndrome which stimulates thyroid adenylate cyclase is not entirely known, it is conceivable that there exist mechanisms independent of TSH to compensate the decreased serum T<jats:sub>3</jats:sub> levels in low T<jats:sub>3</jats:sub> syndrome.</jats:p>