A Direct Free Thyroxine (T4) Immunoassay with the Characteristics of a Total T4 Immunoassay

Free thyroxine measurement in clinical practice: how to optimize indications, analytical procedures, and interpretation criteria while waiting for global standardization

Thyroid dysfunctions are among the most common endocrine disorders and accurate biochemical testing is needed to confirm or rule out a diagnosis. Notably, true hyperthyroidism and hypothyroidism in the setting of a normal thyroid-stimulating hormone level are highly unlikely, making the assessment of free thyroxine (FT4) inappropriate in most new cases. However, FT4 measurement is integral in both the diagnosis and management of relevant central dysfunctions (central hypothyroidism and central hyperthyroidism) as well as for monitoring therapy in hyperthyroid patients treated with anti-thyroid drugs or radioiodine. In such settings, accurate FT4 quantification is required. Global standardization will improve the comparability of the results across laboratories and allow the development of common clinical decision limits in evidence-based guidelines. The International Federation of Clinical Chemistry and Laboratory Medicine Committee for Standardization of Thyroid Function Tests has undertaken FT4 immunoassay method comparison and recalibration studies and developed a reference measurement procedure that is currently being validated. However, technical and implementation challenges, including the establishment of different clinical decision limits for distinct patient groups, still remain. Accordingly, different assays and reference values cannot be interchanged. Two-way communication between the laboratory and clinical specialists is pivotal to properly select a reliable FT4 assay, establish reference intervals, investigate discordant results, and monitor the analytical and clinical performance of the method over time.

Comparison of free thyroxine measurement by chemiluminescence and equilibrium dialysis following 131I therapy in hyperthyroid cats

OBJECTIVES

The first objective was to assess correlation between free thyroxine (fT4) measurements by equilibrium dialysis (fT4ED; Antech Diagnostics) and a chemiluminescent enzyme immunoassay (fT4CEIA; IMMULITE 2000 Veterinary Free T4 [Siemens Healthcare Diagnostics Products]) in hyperthyroid, otherwise healthy, cats before (T0), and 1 month (T1) and 11-23 months (T2) after radioactive iodine (131I) therapy. The second objective was to determine correlation between thyroid status based on fT4 (by both techniques) and the gold standard, thyroid scintigraphy.

METHODS

Thyroid status, including thyroid-stimulating hormone (TSH), total thyroxine (TT4) and fT4 serum concentrations, were assessed in 45 client-owned hyperthyroid cats before (T0), and 1 month (T1) and 11-23 months (T2) after 131I therapy. fT4 was determined by a chemiluminescent enzyme immunoassay (CEIA) and equilibrium dialysis (ED). Quantitative thyroid scintigraphy (with sodium 99m-Tc-pertechnetate) was performed at T2.

RESULTS

Spearman correlation between fT4CEIA and fT4ED was 0.81, 0.88 and 0.79 at T0, T1 and T2, respectively. fT4CEIA was consistently lower than fT4ED, with a median difference of -5.4 pmol/l (P <0.001) and -4.9 pmol/l (P <0.0001) at T1 and T2, respectively. At T2, all cats were identified as euthyroid based on thyroid scintigraphy. None of the cats were identified as being hypothyroid, based on serum TT4 and TSH measurements. Nine of 22 (40.9%) cats had an fT4CEIA below the reference interval (RI) at T2, whereas only 2/22 (9.1%) cats had an fT4ED concentration below the RI at T2.

CONCLUSIONS AND RELEVANCE

Good correlation exists between both assays at T1 and T2, but a significant systematic difference is noted at both time points. This could be an indication for reconsideration of the current RI, although further studies are warranted for assessing test accuracy (in otherwise healthy cats and cats with non-thyroidal illness). At this time, routine use of fT4CEIA after 131I therapy is not advised in feline patients.

Determination of free thyroid hormones

Timely diagnosis and treatment of thyroid dysfunction is compelling given the prevalence and severity of the disease. It requires reliance on adequate laboratory testing of serum TSH as a hallmark in combination with free thyroxine/triiodothyronine. Free hormone methods have to accommodate variations in the concentration and binding capacity of binding proteins. This is a challenge because none of the methodologies developed so far measures the actual unbound hormone in serum. The indirect methods provide an approximation while the direct ones estimate the free hormone concentration either in the presence of the protein-bound counterpart, or after physical separation of the free from bound fraction. The ongoing controversy on the validity and lack of comparability of methodologies points to their imperfectness to reflect real in-vivo free hormone concentrations. Therefore, laboratories and clinicians should know the window of validity and limitations of their methods. The recently developed reference measurement system is a key advance towards improved standardization and clinical validity of free thyroid hormone measurements.

The expanding role of tandem mass spectrometry in optimizing diagnosis and treatment of thyroid disease

This review discusses the state-of-the-art measurement of free and total thyroid hormones in clinical laboratories. We highlight some of the limitations of currently used immunoassays and critically discuss physical separation methods for the measurement of free thyroid hormone. Physical separation methods, such as equilibrium dialysis or ultrafiltration, followed by tandem mass spectrometry for the measurement of free thyroid hormones offer many advantages, which we feel, can deepen our understanding of thyroid hormone metabolism and improve patient diagnosis and care. Problems with direct analogue immunoassay methods for FT4/FT3 as well as immunoassay methods for total T3 at low T3 concentrations and during pregnancy are highlighted. Improved diagnosis and patient management can be achieved utilizing tandem mass spectrometry for these measurements.

Evaluation of dysthyronemia in endocrinological patients

Dysthyronemia is the state of the thyroid gland in which the concentration of thyrotropin (TSH) in circulation is within the reference range, but the concentrations of free or total fractions of thyroid hormones (TH) are outside the reference range. Normal values of TSH and increased values of TH are referred to as hyperthyroxinemia or hypertriiodothyroninemia, while normal values of TSH and decreased values of TH are called hypothyroxinemia or hypotriiodothyroninemia.

Thyroid diagnostic tests were carried out at the Institute of Endocrinology, Prague, Czech Republic, in 1999–2009 using the immunoanalytical systems of Roche Diagnostics GmbH, Germany (Elecsys 2010, Modular E170).

Hyperthyroxinemia was found in 6.74% of all parallel sets of TSH and free thyroxine (FT4, n=259,590) values. Hypotriiodothyroninemia was observed in 8.48% of all parallel sets of TSH and total triiodothyronine (TT3, n=73,143). The occurrence of hyperthyroxinemia (TSH-FT4) and hypotriiodothyroninemia (TSH-TT3) was >3 times higher than the occurrence of dysthyronemia for the combinations TSH-FT4 (hypothyroxinemia), TSH-TT4 (total thyroxine, n=1996), TSH-FT3 (free triiodothyronine, n=94,090), and TSH-TT3 (hypertrijodthyroninemia), and >5 times higher in comparison with the combinations TSH-FT4-FT3 (n=93,683), TSH-FT4-TT3 (n=72,373), TSH-FT3-TT3 (n=2466), TSH-TT4-TT3 (n=1779), TSH-FT4-TT4 (n=1571), and TSH-FT3-TT4 (n=1466).

In light of our patient types, we are assuming that the observed hypotriiodothyroninemia (TSH-TT3, 8.48%) is due to a decreased concentration of thyroid binding globulin in postmenopausal women and that hyperthyroxinemia (TSH-FT4, 6.74%) is caused mainly by endogenous autoantibodies against thyroxine in patients with thyroid autoimmune diseases.

Diagnosis of Thyroid Disease: Principles and Problems

Conceptually, thyroid disorders can be classified into four groups, namely: 1. disorders of thyroid morphology, 2. disorders of thyroid function, 3. presence of thyroid autoimmunity, and 4. diagnosis and follow-up of thyroid carcinoma. Of course, these groups are non-exclusive, and often there is overlap between the groups. Ultrasound exam is a standard for the diagnosis of the disorders of thyroid morphology. To diagnose disorders of thyroid function TSH and thyroid hormones should be measured. Presence of thyroid autoimmunity is confirmed by measuring antibodies against thyroid-specific antigens. Thyroid peroxidase (TPO), thyroglobulin (Tg) and TSH receptors antibodies are used in the diagnosis, follow-up and prognosis of autoimmune thyroid disorders. The measurement of serum thyroglobulin has no role in the diagnosis of thyroid cancer, but it is used in the follow-up of patients treated for differentiated thyroid carcinoma of the follicular epithelium. Medullary thyroid cancer (MTC) produces calcitonin and carcinoembryonic antigen (CEA), but calcitonin is specific for MTC. In subjects with MTC, genetic testing should be done, and in positive cases family screening is necessary.

Therapeutic drug monitoring during pregnancy and lactation: thyroid function assessment in pregnancy-challenges and solutions

The diagnosis and monitoring of thyroid disease necessitates the knowledge of thyroid pathophysiology and of the technical limitations of current thyroid-related biochemical tests. Thyroid disease diagnosis and monitoring are further complicated during pregnancy and lactation, due to pregnancy-related changes in thyroid hormone metabolism. Dramatic changes that occur in thyroxine and triiodothyronine ranges during pregnancy pose challenges for hypothyroid gravidas. Very early in pregnancy, levothyroxine replacement needs to be increased. Moreover, increases in thyroid hormone replacement need to be conducted individually and on a timely basis. For reasons that are still not entirely clear, although dependent in part on changes in thyroxine binding, free thyroxine (FT4) levels decrease as pregnancy progresses necessitating the use of trimester-specific reference intervals for appropriate replacement. Thyroxine binding protein levels vary by hormonal status, inheritance, and disease states and are higher in pregnancy; hence, FT4 assays became popular because they measure the unbound hormone. However, current FT4 immunoassays are estimate tests that do not reliably measure FT4 and are known to be sensitive to alterations in binding proteins and therefore are method-specific. The need to reliably identify hypothyroxinemic pregnant patients, especially in the first trimester, is of prime importance for early fetal brain development before the fetal thyroid functions. This article addresses 1) the current limitations of laboratory-free thyroxine immunoassay methodologies and especially during pregnancy; 2) trimester-specific reference intervals for thyroid function tests; and 3) the study of levothyroxine pharmacokinetics in pregnant and nonpregnant women.

New cases of isolated congenital central hypothyroidism due to homozygous thyrotropin beta gene mutations: a pitfall to neonatal screening

BACKGROUND

Congenital central hypothyroidism (CCH) is a rare condition that is often diagnosed in late childhood in countries where neonatal screening programs rely solely on detecting thyrotropin (TSH) elevation. TSHbeta gene mutation is one of the causes of CCH. We describe two cases of c.Q49X mutation and three cases of c.C105Vfs114X mutation in exon 3 of the TSH beta-subunit gene.

SUMMARY

We found two different TSHbeta gene mutations in two families. In one family, we identified a missense mutation in exon 3 leading to a premature stop at position 49 (c.Q49X) in the two affected twins. In the other family, the three affected siblings had a 313delT nucleotide deletion leading to a frame shift responsible for premature termination at codon 114 (c.C105Vfs114X); neonatal screening showed very low TSH levels in all three patients. The presence of inappropriately low TSH levels at birth in the three affected members of the second family raises questions about the value of the TSH level for CCH screening.

CONCLUSIONS

The marked phenotypic variability in patients with the c.Q49X mutation suggests modulation by interacting genes and/or differences in the genetic background. TSHbeta gene mutations should be suspected in neonates with inappropriately low TSH levels.

Analog-Based Free Testosterone Test Results Linked to Total Testosterone Concentrations, Not Free Testosterone Concentrations

Background: Analog-based free testosterone test results, sex hormone binding globulin (SHBG) concentrations, and total testosterone concentrations are somehow related. This study used new experiments to clarify these relationships.

Methods: An analog-based free testosterone immunoassay and a total testosterone immunoassay were applied to well-defined fractions of serum testosterone. First, they were applied to the 2 fractions (retentate and dialysate) of normal male serum obtained by equilibrium dialysis. Second, they were applied to covaried concentrations of SHBG and total testosterone. Third, they were applied to decreasing concentrations of SHBG and protein-bound testosterone, offset by increasing concentrations of protein-free testosterone, while total testosterone was held constant.

Results: The analog-based free testosterone assay and the total testosterone assay detected and reported serum testosterone test results from serum retentate, whereas neither assay detected the free testosterone in serum dialysate. Test results reported by the analog-based free testosterone assay followed varied concentrations of SHBG and total testosterone. When total testosterone was held constant, however, analog-based free testosterone test results did not follow varied concentrations of serum proteins or of free testosterone.

Conclusion: An analog-based free testosterone immunoassay reported free testosterone test results that were related to total testosterone concentrations under varied experimental conditions. This alleged free testosterone assay did not detect serum free testosterone (the test results it reported were nonspecific) and should not be used for this purpose.

Testing for hypothyroidism in dogs

Hypothyroidism is the most common endocrinopathy in the dog. Rather than being a comprehensive review of all possible thyroid function tests, the focus in this article is on the logical progression of test choice, highlighting total thyroxine, free thyroxine, triiodothyronine, thyrotropin (TSH), and antithyroid antibodies. This article includes extensive discussion of the current status of the canine TSH assay and the potential for improving this assay.