The evaluation of a chemiluminescent assay for free thyroxine by comparison with equilibrium dialysis in clinical samples.

We have investigated the diagnostic performance of a chemiluminescence immunoassay for free thyroxine (Ciba Corning Magic Lite) in comparison with equilibrium dialysis. Agreement between the methods was satisfactory across a wide range of samples including those from patients with non-thyroidal illness. The results suggest that the chemiluminescence immunoassay is not subject to the artefacts of widely used analogue tracer methods for free thyroxine estimation.

Free thyroxine, free triiodothyronine, and thyrotropin concentrations in hypothyroid and thyroid carcinoma patients receiving thyroxine therapy.

Free thyroxine (FT4) and free triiodothyronine (FT3) concentrations in serum were measured by direct equilibrium dialysis methods in patients receiving thyroxine replacement or suppression therapy. Four of 50 hypothyroid patients euthyroid on replacement therapy (mean thyroxine dose 120 micrograms/day) had supranormal FT4 concentrations, whereas the FT3 concentrations were normal in all. Forty-one of 56 operated thyroid carcinoma patients on suppressive therapy (mean thyroxine dose 214 micrograms/day) had raised FT4 concentrations, whereas the FT3 concentrations was elevated in only one patient. There was a large difference in mean FT4 values for hypothyroid and thyroid carcinoma patients (17.2 vs 29.5 pmol/l), whereas the difference in mean FT3 values was small (5.0 vs 6.1 pmol/l), suggesting a decreased peripheral conversion of T4 to T3 with increasing concentrations of FT4. Serum TSH concentrations, as determined by an immunoradiometric assay, varied from less than 0.02 to 11.9 mU/l in treated hypothyroid patients; 21 patients (42%) had values outside the reference limits. As a single test, serum TSH is therefore not very useful for the assessment of adequate thyroxine dosage in patients with primary hypothyroidism. In thyroid carcinoma patients, the TSH concentrations were less than 0.18 mU/l; 45 patients had values less than 0.02 mU/l indicating sufficient suppression of TSH secretion in the majority of cases. On the basis of these results we recommend the combination of FT3 and TSH tests for monitoring thyroxine replacement and suppression therapy. FT4 appears less useful than FT3 for this purpose even if special reference values values were adopted for each patient group.

Performance of direct equilibrium dialysis and analogue-type free thyroid hormone assays, and an immunoradiometric TSH method in patients with thyroid dysfunction.

Direct equilibrium dialysis and analogue-type radio-immunoassays for free triiodothyronine (FT3) and free thyroxine (FT4) in serum were compared in 168 subjects with various states of thyroid function. A good diagnostic efficacy for FT3 and FT4 by either type of assay was observed in hyperthyroidism. In hypothyroidism the free thyroid hormone assays, particularly the FT3 assays, performed diagnostically less well, partly because patients with mild disease were included in the study. No significant differences in the percentages of misclassifications of thyroid dysfunction patients by corresponding dialysis and analogue assays were found. We observed a good linear correlation between dialysis and analogue methods for FT3 (r = 0.98) and FT4 (r = 0.97) in this study comprising out-patients not suffering from severe non-thyroidal disease, known from earlier studies in this and other laboratories to interfere in these assays. It is concluded that analogue assays may be used on out-patients in whom severe systemic diseases are less frequent than in hospitalized patients. There are, however, other limitations to the use of analogue assays than systemic diseases. We observed two euthyroid patients with thyroxine auto-antibodies causing very high FT4 concentrations as determined by analogue assay; their dialysable FT4 concentrations were normal. We also tested a recently developed immunoradiometric serum TSH assay, which was found to perform well in primary hypo- and hyperthyroidism. Serum TSH was elevated in one patient hyperthyroid because of a TSH-producing pituitary adenoma, and within the reference limits in a patient with secondary hypothyroidism.

Concentrations of iodothyronines in serum of patients with chronic renal failure and other nonthyroidal illnesses: role of free fatty acids.

The mean concentration of free thyroxin (FT4) in serum, as determined by direct equilibrium dialysis, was decreased in patients with chronic renal failure (CRF) and increased in patients with various other nonthyroidal illnesses (NTI). The mean concentration of dialyzable free triiodothyronine (FT3) in serum was equally low in both groups of patients. Patients with CRF of various etiology but a similar degree of renal failure as estimated from serum creatinine assay had very similar concentrations of FT4 and FT3 in their serum. Mean thyroxin (T4) and triiodothyronine (T3) concentrations in serum were decreased in CRF and NTI, whereas the mean reverse-T3 concentration in serum was normal in CRF and increased in NTI. T4-binding globulin and albumin were markedly decreased in CRF and NTI; T4-binding prealbumin was increased in CRF and decreased in NTI. The mean concentration of nonesterified free fatty acids (FFA) in serum was increased in NTI but not in CRF. The weak, but significant, positive correlation observed between FT4 and FFA in serum (r = 0.34, P less than 0.01) in NTI indicates that the increase in serum FT4 in this group of patients could be an effect, at least in part, of FFA competing with T4 for binding sites on serum proteins. The stronger correlation detected between the serum FT4 concentration and the FFA/albumin molar ratio in serum (r = 0.60, P less than 0.001) demonstrates the importance of a low albumin concentration for expression of the effect of FFA on FT4 in severe systemic illnesses.