1992 Volume 39 Issue 6 Pages 525-531
To clarify whether serum free fatty acid (FFA) is an inhibitor of extrathyroidal conversion (IEC) of thyroxine (T4) to thyronine (T3), we measured the concentration of FFA, IEC activity and thyroid hormones in normal subjects, acute ketotic children and children with low T3 syndrome due to nonthyroidal illness (NTI). Iodothyronine (I) 5'-deiodinase activity was assayed with reverse triiodothyronine (rT3) as substrate and liberated 125I-was measured. The IEC was determined by the inhibition of I 5'-deiodination by ether extract of sera or standard oleate solution. IEC values were represented as mM oleate. The serum concentration of FFA was 0.470±0.117 (SD) mM in 11 normal subjects, and it was significantly higher (1.242±0.248mM; P<0.01) in 10 acute ketotic children and in 7 samples from 6NTI children (0.904±0.530mM; P<0.05). In contrast, there was no difference in IEC among three groups (normal subject, 0.451±0.069mM; acute ketosis, 0.437±0.040mM; NTI, 0.465±0.224mM). No correlations were found between IEC activity and the serum FFA concentration or thyroid hormones in 28 samples from three groups. The sequential changes in serum thyroid hormones, FFA and IEC in 3 of 6 NTI children revealed no consistent relationship. Furthermore, one NTI child had significantly high IEC (>1.000mM) but its serum FFA (1.182mM) was below the mean value for the acute ketotic group. These results indicate that 1) many NTI patients may bear no relation to IEC and 2) IEC may not be caused by serum FFA only but includes several factors.
