[Thyroxine (T4) and tri-iodothyronine (T3) determinations: techniques and value in the assessment of thyroid function]. / Dosages de thyroxine (T4) et tri-iodothyronine (T3): techniques et place dans le bilan thyroïdien fonctionnel.

Hormonal production of the thyroid gland is constituted of thyroxine or T4 (80%) and triiodothyronine or T3 (20%). In the circulation, whole T4 originates from thyroid secretion but most of T3 (80%) is produced extrathyroidally from T4 deiodination. Conversion of T4 to T3 may be influenced by various conditions and circulating T3 is a less reliable reflection of thyroid hormone production than T4. In serum most of T4 and T3 is bound to binding proteins and only 0.02% of T4 and 0.3% of T3 is free. Because of their higher diagnostic performance, free T4 (FT4) and free T3 (FT3) measurements have superseded total (free + bound) hormone determination. Total hormone measurements remain useful for research studies or in case of severe hyperthyroidism. Equilibrium dialysis/RIA is considered as the reference method for free hormone measurements. Routine clinical laboratories use automated direct two-step or one-step immunoassays with a high molecular weight ligand or labelled antibody. Free hormone measurement remains technically demanding, especially in sera from severe non-thyroid ill patients with low serum thyroxine binding capacity. Interference from anti-thyroid hormone antibodies and familial dysalbuminemic hyperthyroxinemia depends on the assay method, but is now less marked and less frequently detected. To be able to correctly interpret the results of an assay, it is necessary to assess its performance in biologically and clinically well-characterised serum samples. FT4, and FT3 measurements, if FT4 is normal and hyperthyroidism suspected, are used to confirm and assess the level of hypo and hyperthyroidism (overt or subclinical). When the thyroidal status is unstable (first months of a thyroid treatment, altered L-T4 dose, subacute thyroiditis) or when the hypothalamic-pituitary function is disturbed (central hypothyroidism), TSH determination is diagnostically misleading and only free hormone measurements are reliable for thyroid function assessment.

Serum thyroxine binding capacity-dependent bias in five free thyroxine immunoassays: assessment with serum dilution experiments and impact on diagnostic performance.

OBJECTIVES: To assess the presence and magnitude of a serum thyroxine binding capacity (sBC)-dependent bias in five free thyroxine (FT(4)) immunoassays, compared with equilibrium dialysis (ED). The exhibited bias is confronted with clinical results from previous studies to evaluate its impact on FT(4) determination in sera with various sBC. DESIGN AND METHODS: The sera of three pregnant women and three non thyroidal ill (NTI) patients were serially diluted in an inert buffer to progressively decrease the sBC. FT(4) values were measured in diluted and undiluted samples with the six assays. RESULTS: As a function of increasing dilution performed on pregnancy sera, except for ED and Vitros ECi FT(4,) the other four FT(4) assay results decreased to different degrees, in the following order: two-step GammaCoat RIA< Elecsys< ADVIA:Centaur< Amerlex-MAB RIA. In sera from NTI patients, the decrease was more marked and found at high dilution with the Vitros ECi assay. Data from previous studies showed that FT(4) measured with biased assays were decreased only in samples from very severely NTI patients with low sBC and that FT(4) results in pregnancy sera with high sBC were not significantly biased. CONCLUSIONS: The dilution test is a sensitive alarm to assess sBC-dependent bias in FT(4) assays. For all FT4 assays and particularly when a bias is observed, documentation should be sought on the diagnostic performance of the assay and supported by a detailed clinical study including samples with low sBC. Physicians should still be educated about the limitations of FT(4) assays.

Thyroid hormone status and nutrient intake in the free-living elderly. Interest of reverse triiodothyronine assessment.

Fasting or hypercaloric diets are established methods of inducing low triiodothyronine (T3) levels that resemble the sick euthyroid syndrome in adults, but little is known on the mechanisms of this syndrome in the elderly. Decreasing T3 does not seem to be an unavoidable consequence of ageing, but the role of illness or other factors in this decline remains unclear. The aim of this work was to study the influence of nutritional factors on thyroid hormone levels in free-living elderly subjects. A 3-day dietary survey was conducted in 440 randomized subjects aged between 65 and 96. Cholesterol, apoproteins, prealbumin, hemoglobin, thyrotropin-free thyroxine (FT4), FT3 and reverse T3 (rT3) were assayed in each subject. Only 11 subjects had low FT3 levels, and they also had low levels of cholesterol, prealbumin and hemoglobin and a lower Folstein score compared to the rest of the population. Twenty-one subjects had isolated elevated rT3 levels, they were older and had significantly lower energy and fat intakes than the rest of the population. There was a clear reduction in FT3 levels and an increase in rT3 levels with age, although in the normal ranges, which occurred despite maintenance of a high-energy intake even in the oldest group. The FT3 level was lower in the subjects with poor health status, whereas high rT3 levels were associated with low energy intake in men. Stepwise regression showed that hemoglobin, age and prealbumin were the best predictors of FT3 levels, whereas age and energy intake were the main predictors of rT3 levels.(ABSTRACT TRUNCATED AT 250 WORDS)

[Ultrasensitive determination of thyrotropin. Improvement in the performance and reduction of the cost of thyroid function tests]. / Dosage ultra-sensible de l'hormone thyréotrope. Amélioration des performances et réduction du coût de l'exploration fonctionnelle thyroïdienne.

The characteristics of an ultrasensitive thyrotropin (TSH) assay method using monoclonal antibodies (TSH-U) were determined in euthyroid subjects, either healthy or with extra-thyroid disease, in treated and untreated hyper-and hypothyroid subjects and in subjects under amiodarone. The thyroid function was evaluated by free thyroxine (FT4) and free triiodothyronine (FT3) assays and by TSH tests and TSH response to TRH. With a sensitivity of 1 and a specificity of 0.94, the TSH-U assay proved highly reliable to evaluate the thyroid function. Dysthyroidism can be excluded when TSH-U levels are normal (0.15 to 4.5 microU/ml). In case of low TSH-U level, measurements of FT4 and FT3 and, if required, thyroid gland scintigraphy are necessary to confirm a diagnosis of hyperthyroidism. In contrast, a high TSH-U level is sufficient for affirm diagnosis of hypothyroidism. The TSH-U assay makes the TRH test redundant and can differentiate between hyperthyroxinaemia with euthyroidism and with hyperthyroidism. It is an effective method to detect disorders in thyroid function and it calls for a re-evaluation of thyroid diagnostic strategy.

[Asymptomatic thyroid dysfunction. Routine detection using the free thyroxine index]. / Dysfonctionnement thyroïdien asymptomatique. Dépistage systématique au moyen de l'index de thyroxine libre.

Free plasma thyroxine index (FT4I) and plasma concentrations of triiodothyronine (T3) were routinely determined on admission in 1862 hospital patients without signs or history of thyroid dysfunction. Total thyroxine (T4) and FT4I values were beyond confidence limits in 16.3% and 7.5% respectively of the patients, whereas T3 values were low in 23%. Among the 84 patients with high FT4I, 31 had clinically unsuspected hyperthyroidism confirmed by the TRH test (T3 increased in 66% of the cases). Among the 46 patients with low FT4I, 14 had demonstrable hypothyroidism (low T3 in 65% of the cases). In this population, the prevalence of proven thyroid dysfunction without clinical symptoms was 1.66% for hyperthyroidism and 0.75% for hypothyroidism. The diagnostic value of normal FT4I was estimated at 50%. The persistence of an abnormal FT4I on a second determination indicated the presence of hyperthyroidism in 72% of patients with high FT4I values and of hypothyroidism in 74% of patients with low FT4I values. In doubtful cases, TSH assays or TRH tests led to the concept of "transient" dysthyroidism, and the potential total prevalence of routinely discovered dysthyroidism could be estimated at 3.54% including 2.47% for hyperthyroidism. The latter occurred in 95% of people older than 50, with a sex ratio of 0.94. The cost of diagnosis for each new case clinically unsuspected hyperthyroidism is 1200 Z, but this could be reduced to 450 Z if only patients over 50 years of age were investigated and if FT4I determinations were replaced by free T4 determinations.

[Simultaneous determination of blood insulin and plasma C-peptide levels. Value during assessment of glucose tolerance, especially in obese subjects (author's transl)]. / Intérêt de la détermination simultanée des taux d'insuline et C-peptide plasmatiques pour l'appréciation de la tolérance glucidique, notamment chez l'obèse.

Insulin secretion is assessed by simultaneous radioimmunological assay of insulin (R.I.I.) and C-peptide (R.I.C.P.) levels under basal conditions, and after stimulation by oral or intravenous glucose administration. Subjects with abnormal glucose tolerance demonstrate an increase in the ratios R.I.C.P. divided by glucose and R.I.C.P. divided by R.I.I. after fasting, increase in R.I.I. and R.I.C.P. occurring later and lasting longer after glucose loading. These anomalies are observed in both obese subjects and those with normal body weights. Simultaneous determination of R.I.I. and R.I.C.P. levels appears of value in a limited number of cases where glucose loading tests do not supply precise information on the quality of glucose tolerance. Obese subjects, in whom abnormal hyperglycemia levels provoked by oral glucose are observed, but in whom the peripheral glucose assimilation coefficient is normal, can be considered to be non-diabetic as shown by the levels of R.I.I. and R.I.C.P. and more particularly by the molar ratio R.I.C.P. divided by R.I.I.