Evaluation of five high-sensitivity American thyrotropin assays

Critical evaluation of fully automated enzyme immunoassays for free thyroxine and thyrotropin

The technical and diagnostic performance of fully automated immunoassays for free thyroxine and thyrotropin using streptavidin/biotin technology (Enzymun-Test) were examined. The between-assay precision for free thyroxine was 10.4%, 5.4%, 2.5%, 2.3%, 1.1%, and 1.8% at 3.02, 6.27, 17.2, 21.9, 25.6, 42.7 pmol/l; and for thyrotropin was 14.2%, 4.7%, 2.9%, 2.8%, 3.2%, 4.5% at 0.12, 0.46, 1.03, 2.05, 4.8, 12.7 mU/l. The functional detection limit of the assay was 0.09 mU/l. Results for the free thyroxine method correlated well with the IMx (r = 0.91) and the equilibrium dialysis (r = 0.95) assay. Results for the thyrotropin method correlated well with the Tandem-TSH (r = 0.99) and the IMx (r = 0.99) assays. The euthyroid reference range was 11-23 pmol/l and 0.5-3.9 mU/l for free thyroxine and thyrotropin respectively. The free thyroxine assay was not influenced by changes in albumin or thyroxine binding globulin concentration but showed increases at oleic acid concentrations > 4 mmol/l. Spuriously elevated free thyroxine concentration were found in 4 patients, due to assay interference by antibodies in the serum. In a follow up study of 46 patients with non-thyroidal illness, serial measurements showed fluctuating free thyroxine and thyrotropin concentrations with abnormal results occurring in 34%. In a hospital setting, a wider range of free thyroxine (10-28 pmol/l) and thyrotropin (0.22-5.9 mU/l) concentration may be observed in patients who are clinically euthyroid. Abnormal thyroid function tests were however transient and follow up resolved most diagnostic problems.

Serum thyrotropin and thyroid hormone levels in elderly and middle-aged euthyroid persons

OBJECTIVE

To determine whether serum thyrotropin (TSH) levels are altered in euthyroid older persons compared with middle-aged adults.

DESIGN

Serum TSH and thyroid hormone levels were measured in a large group of older persons (> 70 years old, n = 216) and their middle-aged offspring (40-60 years old, n = 211) after excluding those with clinical or historical evidence of thyroid disease or abnormal thyroid function.

MEASUREMENTS

Serum TSH, thyroxine (T4), free T4 index, estimated free T4, triiodothyronine (T3), estimated free T3, and ferritin levels were measured on the Abbott IMx instrument. Peroxidase and thyroglobulin antibodies were measured by radioimmunoassay using Kronus kits.

MAIN RESULTS

Overall, serum TSH showed a log-normal distribution. The geometric mean TSH (mU/L) and 95% confidence limits in the older persons, 1.24 (0.29-5.4), did not differ significantly from that in the middle-aged, 1.45 (0.54-3.9). The mean TSH in the 264 women, 1.37 (0.34-5.5), was similar to that of the 163 men, 1.30 (0.48-3.5). The mean TSH in older women, 1.21 (0.22-6.6), was slightly but significantly lower than that in middle-aged women, 1.52 (0.55-4.2). However, when euthyroid women with positive antibodies were excluded, this difference was not significant. Four of the 123 older women had TSH < 0.1 mU/L, but none of the men or middle-aged women had a suppressed serum TSH. The mean TSH in older men, 1.28 (0.43-3.8), was similar to that in middle-aged men, 1.32 (0.55-3.2). Free T4 was slightly higher in older women than middle-aged women. There were no significant correlations between TSH and any thyroid hormone level. Serum ferritin, measured as a potential marker for the action of thyroid hormone, did not correlate with any measure of thyroid function. At least one antibody level was > 10 U/mL in 14.6% of older women, 15.6% of middle-aged women, 4.3% of older men, and no middle-aged men. When those with milder elevations of antibody levels were included (at least one level > 1 U/mL), the prevalence was 32% of older women, 43.3% of middle-aged women, 15% of older men, and 11.4% of middle-aged men.

CONCLUSIONS

Euthyroid older persons have about the same levels of serum TSH as younger ones, although older euthyroid women have a slightly lower serum TSH than middle-aged women. We recommend that the normal range of serum TSH in the elderly be considered to be the same as that in healthy middle-aged subjects.

Thyroid stimulating hormone and prolactin responses to thyrotropin releasing hormone in nondepressed alcoholic inpatients

Thyroid stimulating hormone (TSH) and prolactin (PRL) responses to thyrotropin releasing hormone (TRH) stimulation are sometimes blunted in alcoholic subjects; however, the mechanisms involved in these phenomena have not been established. We hypothesized that elevations in free thyroid concentrations might be related to these abnormal responses and then tested that hypothesis in a sample of nondepressed alcoholic inpatients (n = 21). Four alcoholic patients had delta max TSH responses that were < 7 mIU/l; three had PRL responses at or below 8 micrograms/l. Baseline TSH was the only significant predictor of peak TSH; however, free thyroxine (FT4) and baseline TSH both were significant predictors of peak PRL. The average baseline FT4 concentration in alcoholic patients was significantly higher than that in healthy control subjects (n = 10). Our data, thus, suggest that small elevations of FT4 play a role in the inhibition of TSH and PRL responses to TRH among nondepressed, abstinent alcoholic patients.

Effective laboratory evaluation of thyroid status

This article focuses on recent developments in thyroid-related laboratory tests, including analytical methods, clinical utility, and limitations of TSH, FT4, T4, FT3/T3, thyroglobulin, and thyroid autoantibodies and the effective use of these tests in the diagnosis of various forms of hypothyroidism or hyperthyroidism, and the management of patients undergoing T4 replacement, T4 suppression, or antithyroid drug therapy.