A comparison of the interference of fatty acids in the competitive binding radioassay and radioimmunoassay for serum T4

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.

Thyroid function tests in thyroid and nonthyroid disease

Modern day evaluation of thyroid disorders requires a combination of accurate clinical judgement and reliable, sensitive, and specific thyroid functions tests. Principle among the latter are thyroxine (T4) 3, 5, 3'-triiodothyronine (T3), and thyroid-stimulating hormone (TSH). Also playing an important role in special situations are free thyroxine, an assessment of bound and unbound thyroid-binding globulin, TRH stimulation, long-acting thyroid stimulator (LATS), antibodies to thyroid hormone and to thyroid receptors. Basic to interpretation of these tests in the clinical setting is a comprehension of the relationship of the hypothalamus, the pituitary, and the thyroid gland as well as a knowledge of the peripheral metabolism of thyroxine and triiodothyronine. The role of each of these laboratory tests in the evaluation of hyper- and hypometabolic states, their alteration in nonthyroid and other endocrine disorders, and the effects of environmental and physiological factors on these tests are reviewed.

Abnormal thyroid function tests in devere non-thyroidal illness: diagnostic and pathophysiologic aspects

In vitro thyroid function tests were studied in twenty-three patients with serious non-thyroidal illness. All had reduced protein binding of serum thyroxine (T4) and serum triiodothyronine (T3) as reflected in increased T4 and T3 uptake tests. The mean T4-binding prealbumin (TBPA) capacity and concentration were about one third the normal levels, whereas the decrease in R4-binding globulin (TBG) was much smaller. Increased serum free fatty acids and reverse T3 were frequently observed, but in vitro displacement of thyroid hormones from their binding sites was achieved only with much high concentrations of these compounds. Other still unrecognized substances significantly inhibiting binding of thyroid hormones might, however, occur in sera of severely ill patients. Evidence in favour of this possibility was the disproportionately high serum T4 by TBG-binding assay relative to T4 by radioimmunoassay. In most of the patients the dual-stage free T4 was elevated, whereas the single-stage free T4 index (CT4I) was within the reference interval. However, neither of these indices reflected the moderately increased dialysable free T4 concentration very accurately. The free T3 index was depressed in most of the patients, whereas the dialysable free T3 concentration was not affected. For practical purposes the combination of normal serum T4 and CT4I in a severely ill patient indicates absence of an associated thyrometabolic disorders.

Mechanisms of interference of nonesterified fatty acids in radioimmunoassays of steroids

Addition of nonesterified fatty acids caused an apparent increase in unbound steroid present in supernatants in radioimmunoassays for steroids utilizing dextran-coated charcoal. Nonesterified fatty acid interference occurred at the initial binding interaction between steroid and its antiserum, and also at the step separating bound from unbound steroid. It was determined that nonesterified fatty acids, which had been added to radioimmunoassays, formed micelles and trapped steroids. The extent of the entrapment was inversely related to the number of polar groups in the steroid molecule, that is, hydrophobic steroids more easily interacted with the nonesterified fatty acid micelles. However, if the charcoal concentration were increased, the nonesterified fatty acid effect was eliminated for assays of polar steroids and greatly reduced for non-polar steroid assays.