New ultrafiltration method for free thyroxin compared with equilibrium dialysis in patients with thyroid dysfunction and nonthyroidal illness.

This new ultrafiltration method for free thyroxin in serum [FT4(U)] is based on radioimmunoassay of the free hormone fraction in ultrafiltrates obtained by centrifuging serum samples in Unisep Ultracent-10 ultrafiltration devices. We compared the results obtained with those by an equilibrium dialysis method [FT4(D)]. In 36 euthyroid healthy subjects, the mean FT4(U) concentration was 24.2 pmol/L and the mean FT4(D) concentration 14.8 pmol/L. In hyperthyroid and hypothyroid patients, results by the ultrafiltration method were also approximately twice as high as those obtained by the dialysis method. In 23 patients with various nonthyroidal illnesses, mean FT4(U) was 41.2 pmol/L and mean FT4(D) 19.8 pmol/L. The mean FT4(U)/FT4(D) ratio in patients with nonthyroidal illnesses (1.97) was not significantly higher than in control subjects (1.68), making it unlikely that the increase in serum FT4 is caused by weakly protein-bound and therefore dialyzable inhibitors of thyroxin binding to carrier proteins. However, two nonthyroidally ill patients with a clearly increased FT4(U) but a normal FT4(D) concentration might have had such inhibitors, whereas for two other nonthyroidally ill patients a high molar ratio of free fatty acids to albumin is a more likely explanation for increased FT4(U) and FT4(D) concentrations. On theoretical grounds, we consider the FT4(U) concentrations analytically more nearly accurate than FT4(D) values for all patient groups studied.

Association between increased concentrations of free thyroxine and unsaturated free fatty acids in non-thyroidal illnesses: role of albumin.

We measured the concentrations of non-esterified free fatty acids and free and total thyroid hormones in serum from patients with various non-thyroidal illnesses (NTI) and chronic renal failure (CRF). The total concentration of free fatty acids was measured enzymatically and the eight most abundant fractions were determined by gas-liquid chromatography. The concentration of total free fatty acids was significantly increased in the NTI group as compared with controls (p less than 0.01); the concentrations of oleic, linoleic and linolenic acid were increased more than those of the other fractions. In NTI the serum-free thyroxine (FT4) concentration was increased (p less than 0.01) and the free triiodothyronine (FT3) concentration was decreased (p less than 0.001); these concentrations were measured by equilibrium dialysis. There was a significant correlation between the levels of total free fatty acids and FT4 in the NTI (n = 43) group (r = 0.45, p less than 0.01), and also between the levels of linoleic acid and FT4 (r = 0.35, p less than 0.05). The serum albumin concentration was decreased in the NTI group, and when free fatty acid to albumin molar ratios were calculated stronger correlations with FT4 were observed (total free fatty acids: r = 0.55; p less than 0.001; oleic acid: r = 0.30, p less than 0.05; linoleic acid: r = 0.46, p less than 0.01; linolenic acid: r = 0.35, p less than 0.05). There was no correlation between FT4 and unsaturated FFA concentrations in CRF patients, who had normal mean FT4 and total FFA levels. These results support the hypothesis that unsaturated fatty acids are involved in the increase of serum FT4 in NTI, especially when albumin levels are low.