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

Abstract

Direct equilibrium dialysis and analogue-type radio-immunoassays for free tri-iodothyronine (FT₃) and free thyroxine (FT₄) in serum were compared in 168 subjects with various states of thyroid function. A good diagnostic erficacy 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 hyper-thyroid because of a TSH-producing pituitary adenoma, and within the reference limits in a patient with secondary hypothyroidism.

Key words:

• free thyroxine
• free tri-iodothyronine
• thyroid disease
• thyrotropin