Anti-triiodothyronine auto-antibody interference in recent free thyroid hormone assays

Evaluation of free thyroxine determination based on one-step fluorometric immunoassay technique and the suboptimal concordance with two-step immunoassay

Free thyroxine (FT4) quantification is continuing to be a concern. The purpose of the following study was to evaluate the analytical performance of Tosoh AIA900 based on a one-step technique and its comparison to Access 2 (two-step technique) over different clinical contexts (euthyroid, thyroid disorders, uncontrolled diabetes, renal failure and pregnancy). The protocol established by the French society of Clinical Biology was used to evaluate: imprecision, limit of detection, trueness, linearity, interferences and method comparisons. Within-run variation of 3.1%, 5.7% and 4.4% were found for the low, medium and high controls, respectively. Between-run was 5.8% for low control, 5.7% for medium control and 7.1% for high control. Common interferences did not affect one-step immunoassay FT4 results. The linearity was checked up to 86 pmol/L. The limit of detection was 5.5 pmol/L. The concordance correlation coefficient (CCC) showed a low agreement (0.6) between both methods. Bland-Altman plot revealed that AIA 900 one-step immunoassay technique provides a significant higher values ((+2.8 ± 2.7 pmol/L;p < 0.0001). The Passing-Bablok regression demonstrated both proportional and systematic differences in comparison to Access 2. The lowest association was noted in subjects with impaired renal function (CCC = 0.27). At the time of the study, the results of on-step immunoassay are not directly comparable with Access 2.

Unexpected Elevated Free Thyroid Hormones in Pregnancy

BACKGROUND

The use of thyrotropin and free thyroid hormone assays to evaluate thyroid function is widespread, but in some situations the results are inconsistent with the patient's thyroid status.

SUMMARY

A 35-year-old woman with a known diagnosis of chronic autoimmune thyroiditis was referred to the authors' clinic at week 26 of her second pregnancy. The patient was clinically euthyroid. Consistent with this, her serum thyrotropin (TSH) was normal (0.79 mIU/L), but she had elevated free thyroid hormones-free triiodothyronine (fT3) and free thyroxine (fT4)-as determined by a one-step chemiluminescent assay. The patient was taking levothyroxine replacement therapy (125 μg/day), and the dose was confirmed. Previous blood tests showed concordance between TSH and free thyroid hormone values. The patient was followed up throughout gestation and at 12 months postpartum. During gestation, her free thyroid hormones remained high using one-step methods, while the total thyroid hormone concentration values were within the reference range, in agreement with the TSH values. Postpartum fT4 and fT3 values returned progressively to normality, in agreement with the TSH values. The presence of circulating thyroid hormone autoantibodies (THAb) was hypothesized, which are known to interfere, although to a variable extent, with thyroid hormone one-step assays. Using stored frozen sera, this hypothesis was confirmed indirectly by measuring normal levels of fT3 and fT4 with a two-step method, and directly by demonstrating THAb against the two hormones.

CONCLUSION

Despite their relative rarity, circulating THAb may be suspected when laboratory data are not consistent and contrast with the clinical picture. To the authors' knowledge, no previous case of transient appearance of THAb in pregnancy has been described.

Interference of anti-T4 autoantibodies in the single step free T4 radioimmunoassays

The thyroid hormone profile of a hypothyroid patient, with moderate sized diffuse goiter and history of exogenous thyroid hormone therapy, was found to be highly ambiguous. The total as well as free T4 levels were consistently high (out of range) with suppressed T3 (total and free) and elevated thyrotropin levels. We incubated the serum with the radio-iodine labelled T4 and observed that the patient's serum bound about 50.5% of the total radioactivity resulting in 85.3% interference in the radioimmunoassay. On resolving the serum proteins by cellulose acetate electrophoresis, the radioactivity was detected in the gamma globulin band. The protein profile showed mildly raised levels of IgG. Therefore it was established that the patient's serum carried T4 reactive antibodies (IgG) which were interfering in the immunoassays. Elevated anti-thyroid microsomal antibodies were also present in the patient's serum. The anti-T4 antibodies were highly specific for T4 and did not show any cross reactivity with the T3 or its analogues used in the estimation of free T3. We stripped the serum by precipitating the immunoglobulins and were successful in estimating the levels of free T4 in the stripped serum by single step radioimmunoassy.

Interferences in hormone immunoassays

Despite the numerous potential interferences that were discussed in this article, immunoassays, in general, are robust measurement systems. There is no practical way to identify specimens a priori that are likely to have immunoassay interference. Therefore, laboratories must rely on communication from clinicians to identify suspicious test values that may be caused by assay interference. After laboratories are alerted, multiple investigations can be undertaken. The common causes of immunoassay interferences (see Box I) are dependent on the analytic format. Competitive assays are most likely to have problems with cross-reactivity, whereas immunometric assays most often have problems with heterophile antibodies. The simple steps to test for dilutional linearity, spiked recovery, heterophile antibody blocking,and testing with alternate technology can help in the evaluations of cases that are suspected to have assay interference.

Successful outcome after surgical management in two cases of the "painful variant" of Hashimoto's thyroiditis

OBJECTIVE

To describe two cases of the rare "painful variant" of Hashimoto's thyroiditis (HT) that were refractory to medical management and in which surgical intervention provided the definitive treatment.

METHODS

We thoroughly review the clinical history as well as the laboratory, imaging, and surgical pathology data in these cases, and follow-up of the clinical response over time is provided. The relevant literature is also discussed.

RESULTS

A 56-year-old woman, who had remotely undergone a left hemithyroidectomy and had been diagnosed with HT, sought further assessment because of neck pain and edema. Treatment with corticosteroids was partially successful but led to the development of Cushing's syndrome. A 32-year-old man had pain and swelling of the thyroid and was diagnosed with HT shortly thereafter. Levothyroxine treatment was unsuccessful. Both patients underwent thyroidectomy. Chronic lymphocytic thyroiditis (HT) with a variable degree of fibrosis was found on assessment of pathology specimens. The patients remained asymptomatic after the surgical procedure and did not require any further anti-inflammatory therapy.

CONCLUSION

In selected cases, surgical treatment may become necessary for effective and permanent control of symptoms and local signs in painful HT. Access to experienced endocrine surgeons is important in order to avoid postoperative complications because the thyroid gland may be small or fibrosed in this rare variant of HT.

Evaluation of Elecsys free triiodothyronine assay: relevance of age-related reference ranges

OBJECTIVE

This study aimed at evaluating Elecsys free triiodothyronine (FT3) assay performed on an Elecsys 2010 system, while paying special attention to age relationship in euthyroid subjects.

DESIGN AND METHODS

FT3 levels were measured in 149 euthyroid control subjects aged between 2 and 92 years old, 33 hyperthyroid and particular euthyroid patients: female in the last 3 months of pregnancy (n = 30), nonthyroidal ill hospitalized in medical (NTlm, n = 31), or intensive care units (NTlc, n = 31) and amiodarone-treated (n = 27).

RESULTS

FT3 was inversely related to age in controls (r = -0.67). Three reference ranges were used: below 20 years 4.5-9.0 pmol/L, between 20 and 60 years 3.9-7.2, and over 60 years 2.4-6.5. Compared to age-matched controls, FT3 decreased in pregnancy, NTlm, NTlc, and amiodarone groups. Use of age-related reference ranges improved the specificity markedly in amiodarone patients and to a lesser extent in NTlm and TClc patients.

CONCLUSIONS

The reliability of the Elecsys FT3 assay was found to be satisfactory for clinical use, when the age of patients was taken into account.

Antibody interference in thyroid assays: a potential for clinical misinformation

Measurements of thyrotropin and of total and free thyroxine and triiodothyronine are widely used diagnostic methods for thyroid function evaluation. However, some serum samples will demonstrate a nonspecific binding with assay reagents that can interfere with the measurement of these hormones. Several recent case reports have described the presence of such interferences resulting in reported abnormal concentrations of thyroid hormones inconsistent with the patient’s thyroid state. Circulating thyroid hormone autoantibodies, described in thyroid and nonthyroid disorders, are an important class of interference factor and can bind to hormone tracers used in various immunoassays. Two additional categories of interfering antibodies may particularly interfere within two-site immunoassays for thyrotropin. These include heterophile antibodies, especially human anti-mouse antibodies, and rheumatoid factors, which can cause interferences by immunoglobulin aggregation and (or) cross-linking of both capture and signal antibodies. Here we review the nature of these disturbances; their occurrence, prevalence, and detection; and the clinical consequences of the failure to recognize such interference.