Analytical comparability of thyroid-stimulating hormone and free thyroxine across capillary and venous collection matrices for remote thyroid diagnostics

Authors: Natalie Getreu https://orcid.org/0000-0003-2572-9992 [email protected], Zoe Grant https://orcid.org/0009-0000-9275-8924, Tharni Vasavan https://orcid.org/0000-0002-3433-9681, Adrian Timpson https://orcid.org/0000-0003-0292-8729, Timothy Woolley https://orcid.org/0000-0001-6728-8174, and Helen C O'Neill https://orcid.org/0000-0003-3481-3459Author Info & Affiliations

Publication: Journal of Patient Centricity

Volume 1, Number 1

https://doi.org/10.57264/jpc-2026-0005

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Abstract

Aim: To evaluate the analytical comparability of thyroid-stimulating hormone (TSH) and free thyroxine (FT4) in capillary blood samples collected using serum separator tubes compared with venous reference standards. Background: Advances in at-home blood collection have enabled individuals to monitor key reproductive hormones with clinical accuracy. However, the hydrophobic nature of serum separating gels in small-volume capillary tubes can cause matrix-related interference with certain lipophilic analytes, such as sex steroids. This study investigates whether TSH and FT4 maintain analytical integrity within this specific collection matrix, a critical requirement for ensuring the accuracy of remote thyroid diagnostics. Materials & methods: This secondary analysis, drew data from two collection events of a validated primary study: an at-home collection setting (Trial 1, capillary samples only) and a supervised clinic-based setting (Trial 2, concurrent capillary and venous samples). Samples from 39 participants were analyzed for TSH and FT4 using the Cobas^® e801 immunoassay platform (Roche Diagnostics). Statistical significance was assessed using paired t-tests on log-transformed values, with a Šidák-adjusted threshold (α = 0.000814) to account for multiple comparisons. Results: TSH and FT4 demonstrated robust analytical comparability across all seven tube-pair comparisons. Mean percentage differences for TSH ranged from -2.96% to +2.43%, and for FT4 from -1.49% to +0.50%. All differences fell substantially below the desirable total allowable error (TEa) thresholds derived from the European Federation of Clinical Chemistry and Laboratory Medicine Biological Variation Database (24.7% for TSH; 6.3% for FT4). In contrast to the pronounced gel-related suppression previously documented for estradiol (-13.3%) and testosterone (-9.0%), thyroid analytes exhibited no clinically significant matrix-related bias. Conclusion: TSH and FT4 demonstrate consistent analytical comparability across capillary serum separator tube collection matrices. These findings provide preliminary analytical evidence supporting capillary-based sampling as an alternative to venipuncture for thyroid monitoring within the pre-analytical conditions studied, and support the integration of these markers into decentralised, at-home diagnostic frameworks.

Plain language summary: What does capillary blood collection mean for thyroid hormone testing at home?

What is this article about?

At-home blood testing services allow people to collect small samples by finger-prick rather than visiting a clinic for a standard blood draw. These samples are collected in small tubes that often contain a gel which separates the liquid part of the blood. Previous research shows that this gel can interfere with the sample, and artificially lower the levels of certain hormones, this was seen in estrogen and testosterone. This study investigated whether the same problem occurs for two thyroid hormones, thyroid stimulating hormone (TSH) and free thyroxine (FT4), which are important for diagnosing and monitoring thyroid conditions.

What were the results?

Samples from 39 participants were collected in four types of blood collection tubes (with and without gel, using both finger-prick and standard blood draw). TSH and FT4 measurements were essentially identical across tube types. The small differences observed were well within the internationally accepted limits for clinical accuracy. Unlike estrogen and testosterone, TSH and FT4 were not affected by the gel.

What do these results suggest?

TSH and FT4 can reliably be measured from finger-prick samples collected in standard gel containing tubes used in at-home testing kits. The gel does not interfere with these thyroid hormones. This means that people with thyroid conditions may be able to monitor their health from home without needing to visit a clinic for a blood draw, making monitoring thyroid treatment more accessible.

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References

Hoffman MSF, McKeage JW, Xu J, Ruddy BP, Nielsen PMF, Taberner AJ. Minimally invasive capillary blood sampling methods. Expert Rev. Med. Devices 20(1), 5–16 (2023).