ASSESSMENT OF CONCENTRATION OF ACETIC-ACID AND PROPIONIC-ACID DERIVATIVES OF 3,5,3'-TRIIODOTHYRONINE IN HUMAN-SERUM

In order to assess, by RIA, the serum concentration of 3, 3́, 5-triiodothyroacetic acid (Triac) and 3, 3́, 5-triiodothyropropionic acid (Triprop), antisera were prepared by immunization of rabbits with conjugates of these analogs with human serum albumin. RIA of both Triac and Triprop involved 1) extraction of Triac and Triprop from sera by methanol-2N NH40H (99:1) and evaporation of extracts to dryness; 2) purification of iodothyronines in the residue using Sephadex (LH-20) column chromatography and subsequent concentration of the eluate; 3) descending paper chromatography in hexane-tertiary amyl alcohol-2N NH40H (1:5:6) to separate Triac (and Triprop) from T3 and T4; 4) elution of chromatogram zones harboring Triac and Triprop with methanol-2N NH40H, evaporation of methanol-NH4OH eluate and reconstitution in a convenient volume of assay buffer (0.075 M barbital buffer); 5) comparison with standards of the ability of the reconstituted extract of the unknowns to inhibit the binding of [131I]Triac or of [131I]Triprop to respective (anti-Triac or anti-Triprop) antisera. Loss of Triac (or Triprop) during extraction was monitored by study of the recovery of radioactive Triac and radioactive Triprop added to each specimen before extraction. The mean recovery of four different concentrations of non-radioactive Triac and Triprop was 54.3% and 56.8%, respectively, before correction for procedural losses, and 90.5 and 90.9%, respectively, after correction. Serum Triac concentration varied between <5.5 and 15.2 ng/100 ml (mean 8.7) in 24 (11 euthyroid, 10 hyperthyroid, and 3 hypothyroid) cases so studied; the values in hyper-and hypothyroid subjects overlapped with those in euthyroid subjects. Serum Triprop concentration was less than the detection threshold of the assay (8.8 ng/100 ml) in all 24 cases referred to above. The value was 2.7 ng/100 ml or less in one euthyroid and three hyperthyroid subjects in whom 5-fold concentrates of serum extracts were employed in the RIA. These data indicate the presence of some (albeit small) amounts of side chain derivatives of T3 in human serum and suggest that deaminative and decarboxylative metabolism of thyroid hormone is operative in man. © 1978 by The Endocrine Society.