Routine free thyroxine reference intervals are suboptimal for monitoring children on thyroxine replacement therapy and target intervals need to be assay-specific

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.

Pelagic distribution of plastic debris (> 500 µm) and marine organisms in the upper layer of the North Atlantic Ocean

At present, the distribution of plastic debris in the ocean water column remains largely unknown. Such information, however, is required to assess the exposure of marine organisms to plastic pollution as well as to calculate the ocean plastic mass balance. Here, we provide water column profiles (0–300 m water depth) of plastic (0.05–5 cm in size) concentration and key planktonic species from the eastern North Atlantic Ocean. The amount of plastic decreases rapidly in the upper few meters, from ~ 1 item/m³ (~ 1000 µg/m³) at the sea surface to values of ~ 0.001–0.01 items/m³ (~ 0.1–10 µg/m³) at 300 m depth. Ratios of plastic to plankton varied between ~ 10^–5 and 1 plastic particles per individual with highest ratios typically found in the surface waters. We further observed that pelagic ratios were generally higher in the water column below the subtropical gyre compared to those in more coastal ecosystems. Lastly, we show plastic to (non-gelatinous) plankton ratios could be as high as ~ 10²–10⁷ plastic particles per individual when considering reported concentrations of small microplastics < 100 μm. Plastic pollution in our oceans may therefore soon exceed estimated safe concentrations for many pelagic species.

Establishing paediatric reference intervals for thyroid function tests in Croatian population on the Abbott Architect i2000

Introduction

Evaluation of thyroid function is often requested and therefore defining paediatric reference intervals (RIs) is of vital importance. Currently, there is a distinct lack of paediatric RIs for thyroid function tests in Croatia. Thus, we established RIs for thyroid stimulating hormone (TSH), total triiodothyronine (TT3), total thyroxine (TT4), free triiodothyronine (FT3) and free thyroxine (FT4) in the Croatian paediatric population.

Materials and methods

Reference intervals were calculated from 397 apparently healthy children, aged from 2 days to < 19 years. Serum samples were analysed for thyroid function tests on the Abbott Architect i2000. Age- and sex-specific 95% RIs with 90% confidence intervals were established according to Clinical and Laboratory Standards Institute guidelines. To express the magnitude of sex and age variation, standard deviation ratio (SDR) was calculated using two-level nested ANOVA. The criterion for considering partitioning reference values was set to SDR > 0.3.

Results

All thyroid function tests required age partitioning, confirmed by SDR above 0.3. There was no need for sex partitioning, confirmed by SDR below 0.3. Still, FT3 was partitioned due to visually noticeable sex related difference for the oldest group (12 years to < 19 years).

Conclusion

This is the first study to establish RIs for thyroid function tests in the Croatian paediatric population. We propose RIs for widely used Abbott platform, thus giving laboratories method- and population-specific paediatric RIs for thyroid function tests that should improve clinical test interpretation.

USE OF THE FREE THYROXINE INDEX TO REFINE THE LOWER LIMIT OF A FREE THYROXINE IMMUNOASSAY FOR DETECTION OF SECONDARY HYPOTHYROIDISM

OBJECTIVE

To determine the utility of measuring free T₄ index (FT₄I) in patients with low free T₄ (FT₄) levels using immunoassay and normal thyroid-stimulating hormone for the evaluation of secondary hypothyroidism.

METHODS

We performed a retrospective medical chart review of patients seen at a single institution as outpatients who had a simultaneously normal thyroid-stimulating hormone level, low FT₄ level, and any FT₄I measured between June 2014 and October 2016. Demographic, laboratory, and imaging data were collected. Using FT₄I as the reference for diagnosis of hypothyroidism, the sensitivity and specificity of the FT₄ immunoassay's lower-limit thresholds were determined. Within each threshold group, available brain imaging and biochemical evaluation were categorized according to the presence or absence of pituitary disease.

RESULTS

A total of 155 sets of result pairs (FT₄ and FT₄I) performed on 118 subjects were analyzed. The lower limit of a normal FT₄ level by immunoassay at this institution was 0.93 ng/dL, though all pairs with FT₄ ≥0.89 ng/dL had a normal FT₄I. All pairs with FT₄ ≤0.67 ng/dL had a low FT₄I. No pituitary macroadenomas were identified in any subject, though the rates of pituitary imaging in this patient sample were low.

CONCLUSION

Patients with a borderline low FT₄ level by immunoassay often have normal FT₄I. In such patients at our center, significant structural and biochemical pituitary pathology was uncommon.

Reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) in infants' day 14-30 of life and a comparison with other studies

Objectives Established reference intervals of thyroid function in neonates are important; however, studies often consist of a small sample size or lack of clinical information. We aim to define reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4) for infants aged 14-30 days. We also reviewed follow-up TSH for infants with initial values 10-20 mIU/L. Methods Venous TSH and FT4 of term babies aged 14-30 days with breast milk jaundice that had thyroid function test performed as part of a prolonged jaundice workout from September 2016 to March 2017 were analyzed. Electronic medical records were reviewed to ensure only well babies with no pathological causes of jaundice or conditions that may affect thyroid function were included. TSH and FT4 were analyzed using immunoassay analyzer Dxl 800, Beckman Coulter. Results There were no correlations between FT4 and TSH with gender, birth weight and ethnicity. Correlation coefficient between FT4 and total bilirubin was weak at 0.138 (p=0.001). No association was found between TSH and bilirubin levels. Mean FT4 was higher in the younger age group day 14-21 (p<0.01). There was no significant difference in TSH values between the age groups. Infants with mildly elevated TSH 10-20 mIU/L had normalized values on follow-up (mean, 11.41 vs. 4.42 mIU/L; p<0.01; 95%CI, 5.88-8.09). The following reference intervals (2.5-97.5th percentile) were derived: FT4 day 14-21 (n=513): 11.59-21.00 pmoL/L; FT4 day 22-30 (n=66): 10.14-19.60 pmoL/L; TSH day 14-30 (n=579): 1.90-10.34 mIU/L. Comparison between studies showed variations of reference intervals with different manufacturer assays, age and methodology. Conclusions Our reference intervals would be useful in the clinical setting. Infants with mildly elevated TSH could be monitored first instead of immediate treatment.