Transthyretin--an explanation of "anomalous" serum thyroid hormone values in severe illness?

The following serum analytes were measured in 464 patients with defined carcinomas and other tumours as well as those with chronic obstructive lung disease and under regular haemodialysis, and in 261 healthy controls: thyrotropin (TSH), thyroxine (T4), triiodothyronine (T3), free thyroxine (fT4), thyroxine binding globulin (TBG) and transthyretin (TTR). The following ratios were constructed: fT4 x TTR, defined as the thyroxine availability index fT4/TTR, defined as the thyroid hormone compensation index 100 x T3/TBG as the free T3 index (fT3I) and fT3I x TTR, defined as the triiodothyronine availability index. Significantly elevated thyrotropin values (p = 0.05) were only found in patients with breast cancer when compared with age matched controls, although elevated T4 and fT4 values were found in all experimental groups except the haemodialysis patients (p less than 0.01). The thyroxine availability index and triiodothyronine availability index values were not significantly different from the age matched controls (greater than 60 a) in the cancer groups, showing that the transthyretin concentrations compensated for changes in fT4 or vice versa. These findings are reflected in the euthyroid thyrotropin values. The T4 and fT4 values in the dialysis patient group were significantly lower than in the age matched controls (p less than 0.01), while the transthyretin values were significantly higher (p less than 0.01), which accounted for the normal thyroxine availability index and euthyroid thyrotropin values.