Serum sex hormone-binding globulin and osteocalcin in systemic nonthyroidal illness associated with low thyroid hormone concentrations.

To test the hypothesis that patients with systemic nonthyroidal illness (NTI) and impaired thyroid hormone metabolism most commonly present with the low T3 or low T4 syndrome are in an euthyroid status at the tissue level, we determined serum sex hormone-binding globulin (SHBG) and osteocalcin (OC) as parameters for thyroid hormone availability to liver and bone. Serum SHBG and OC concentrations were measured in 61 severely ill patients with decreased serum levels of either T3 alone (n = 47) or both T3 and free T4 (n = 14). None of the patients had primary thyroid disorder indicated by regular thyroid sonomorphology and normal basal TSH concentrations. Data were compared with values obtained from age- and sex-matched controls without impairment of thyroid hormone physiology. The respective results from the study groups and from control subjects were as follows: low T3 SHBG, 49 +/- 30 nmol/L; low T4 SHBG, 35 +/- 14 nmol/L; control SHBG, 45 +/- 28 nmol/L; low T3 OC, 0.87 +/- 0.75 nmol/L; low T4 OC, 0.75 +/- 0.71 nmol/L; control OC, 0.98 +/- 0.87 nmol/L. SHBG and OC do not differ significantly between NTI patients with low T3 or low T4 syndrome and controls, and no significant relationship was found between thyroid hormone parameters and SHBG or OC, respectively. We conclude that the term euthyroid sick syndrome applied for the condition of systemic NTI associated with reduced circulating thyroid hormone concentrations appears to be appropriate at the level of hepatocytes and osteoblasts. Data are in partial contradiction to the results of previous studies using alternative serum parameters for assessment of peripheral thyroid hormone action (e.g. angiotensin-converting enzyme). Due to the tissue specificity of the biochemical serum markers, conflicting results may be explained by organ-specific differences in local thyroid hormone exposure. In case of doubt, SHBG and OC determinations may help specify peripheral thyrometabolic status in NTI.