Four radioisotopic immunoassays of free thyroxine compared

The analytical and clinical performance of three radioisotopic methods for the measurement of free thyroxine-Gamma Coat (125I) Free T4, RIA-gnost FT4 and Amerlex-MAB* FT4-were assessed in comparison with the Scalvo two-step chromatographic method. The analytical evaluation indicated excellent performance of Amerlex-MAB FT4. For the other methods, precision and sensitivity were comparable and acceptable for routine use. Only Gamma Coat FT4 showed a significant positive intra-assay drift. None of the methods showed a significant correlation with thyroxine-binding globulin. Amerlex-MAB FT4 results were weakly positively correlated with albumin. Sclavo, RIA-gnost and particularly GammaCoat FT4 results were affected by the presence of increased concentrations of free fatty acids. All methods classified hyperthyroid patients correctly. Slight overlaps existed between the hypothyroid and euthyroid populations. Significant decreases in free thyroxine (FT4) during the third trimester of pregnancy were detected by all assays. In patients with chronic renal failure, serum FT4 was within reference limits more often when measured by Sclavo or Amerlex-MAB methods than by RIA-gnost or GammaCoat techniques. In conclusion, all assays performed well from both technical and diagnostic points of view.

Factors affecting the serum free thyroxine levels in hospitalized chronic geriatric patients

OBJECTIVE

Determination of whether nonthyroidal factors affect the diagnostic value of free thyroxine estimation in geriatric patients.

DESIGN

Survey.

PARTICIPANTS

A convenience sample of 381 non-selected, chronic, hospitalized geriatric patients over 60 years of age (I = relatively good health; II = relatively poor health; III = bad health; subgroups "sine therapia," ie, patients receiving no drugs that affect FT4) and 180 20-40 year old healthy persons.

MEASUREMENTS

Thyrotropin-releasing hormone test; thyrotropin (TSH); free thyroxine (FT4, measured in part by two parallel methods) estimation in a screening study; and thyroxine-binding globulin and thyroxine-binding-inhibitor activity measurements.

RESULTS

The normal FT4 ranges of the euthyroid geriatric (n = 210) and healthy young groups were similar. In the "sine therapia" euthyroid patients, FT4 decreased with age but increase with the severity of illness. High FT4 levels with non-suppressed TSH were more frequent in patients in poor and bad health. (I = 6/112; II = 14/140; III = 13/74; P < 0.01). The serum thyroxine-binding-inhibitor activity of euthyroid geriatric patients correlated with the severity of their clinical state (I = 6.22 +/- 5.65 (13); II = 7.40 +/- 4.33 (23); III = 10.04 +/- 5.50 (16) micrograms merthiolate equivalent/microL; ANOVA with log-transformed values: F(2.51) = 3.50, P < 0.05). The mean FT4 was higher in 36 heparin-treated patients (22.81 +/- 4.67 pmol/L) than in the 193 "sine therapia" patients (19.03 +/- 4.23 pmol/L; -P < 0.001). In a convenience subsample of 240 patients, a weak inverse correlation was found between FT4 and the thyroxine-binding globulin (r = -0.14, P < 0.02). Only 5/11 patients with low free thyroxine had hypothyroidism, while 11/46 patients with elevated free thyroxine had hyperthyroidism.

CONCLUSIONS

There is no need to modify the normal free thyroxine range for hospitalized geriatric patients. Clinical condition, drug treatment, and, to a lesser extent, age are factors that significantly affect the diagnostic value of FT4 in hospitalized chronic geriatric patients, decreasing the specificity of the test in diagnosing clinical hyper- and hypothyroidism.

Circulating TSH levels measured with an immunochemiluminometric assay in patients taking drugs interfering with biochemical thyroid status

Serum TSH was measured using a high sensitivity immunochemiluminometric assay (ICMA) in patients receiving anticonvulsant drugs, heparin or non-steroidal anti-inflammatory agents (NSAIs) and the results compared with those from groups of age- and sex-matched controls. The TSH results have also been compared with those of estimates of free thyroid hormone levels using Amerlex analogue method reagents. All patients were clinically euthyroid and TSH concentrations were in the normal, euthyroid range and did not differ significantly in any group studied. In contrast, Amerlex free T4 levels were significantly reduced in the patients treated with phenytoin (P less than 0.001), carbamazepine (P less than 0.01), sodium valproate (P less than 0.01) and heparin (P less than 0.001). Patients treated with NSAI agents showed no significant change in free T4 levels. Amerlex free T3 levels were also significantly reduced in the patients treated with phenytoin (P less than 0.005), carbamazepine (P less than 0.005) and heparin (P less than 0.001) but not in those treated with sodium valproate or NSAIs. These data support the view that measurement of circulating TSH with an assay of sufficient precision in the relevant range provides a reliable way of assessing thyroid status and could be used to exclude hyperthyroidism in patients taking the medications investigated in this study.

Recent advances in routine thyroid function testing

Thyroid function tests are one of the most common of endocrine laboratory investigations requested by general clinicians. The tests used therefore have to be efficient at identifying thyroid disease, monitoring treatment, and handling large numbers of tests. Recent advances in methodology have expanded both the range of in vitro thyroid function tests available and the techniques by which the well-established tests may be performed. This article reviews the methods and analytical and clinical performance of the routine tests currently available, concentrating particularly on the relatively new ones, and speculating on their role in strategies for the laboratory investigation of thyroid function.

Effect of heparin on serum free thyroxine linked to post-heparin lipolytic activity

A possible association between free T4 (fT4) changes, occurring after heparin administration, and the post-heparin lipolytic activity, i.e., the release of lipases and the lipolysis of triglycerides to non-esterified fatty acids (NEFAs) was studied. In 19 patients heparin increased mean fT4 values obtained by equilibrium dialysis from 23.4 to 38.6 pmol/l (P less than 0.01) and mean fT4 values measured by GammaCoat two-step RIA from 17.0 to 24.1 pmol/l (P less than 0.01), the results of the two methods agreeing well (rs = 0.88). Concurrently, NEFAs increased from a mean of 0.55 to 2.20 mmol/l (P less than 0.01) and fT4 increases were significantly correlated with post-heparin NEFAs (rs = 0.69, P less than 0.001). In vitro addition of palmitic acid to sera increased fT4 concentrations to values similar to those observed in vivo. Post-heparin fT4 increases were also strongly correlated to the concentrations of triglycerides (rs = 0.63, P less than 0.01). Hypertriglyceridaemia was associated with pronounced fT4 increases after heparin administration, and normolipidaemia with moderate or no changes. These results suggest that the effect of heparin on fT4 is linked to the activation of lipases by heparin and is mediated by NEFAs, which, at high concentrations, compete with thyroxine for binding proteins.

Factors influencing serum free T4 in 70-year-old men. Implications for reference intervals in the elderly

We tested a procedure to obtain reference values based on careful clinical analysis of a population sample with the background (a) that there are conflicting reports of age effects on thyroid function parameters, possibly reflecting differences in numbers of sick and drug-treated subjects in the studied populations, and (b) that recommended procedures for the selection of reference subjects in elderly populations may give unacceptably high exclusion rates. Serum free T4 was determined by a ligand-analogue technique in a representative population of 460 males at age 70. The determinations were performed 5 years after the clinical study, and the results were analysed in relation to information obtained at the study as well as during a 5-year follow-up. The free T4 concentration was significantly low in subjects with neoplastic disease and subjects with chronic bronchitis and significantly high in subjects taking non-selective beta-adrenergic blockers. Selective beta 1-blockers had no apparent effect. Information on death during the 5 years after the study proved to have no significance. The free T4 concentration was found to be negatively correlated to body mass and body mass index, a correlation which has previously not been shown. Smoking was associated with a significantly low body mass but had no apparent effect on mean free T4 concentration. A procedure with successive exclusions of clinically defined diagnostic groups with significantly low or high analyte concentration and/or body mass may be used to obtain reference values for thyroid function and may reveal previously unknown relationships.