Adding interpretative comments to results of thyroid function tests from patients on thyroxine replacement does not improve management.

Aims: To assess the impact of adding clinical comments to reports of thyroid function testing in patients treated for hypothyroidism.

Methods: We compared thyroid function test results in primary care patients being treated for hypothyroidism from January 2016 to August 2023 at two NHS Trusts with similar demographics and using the same instruments, but with different interpretative comment policies. One laboratory, Buckinghamshire Health Trust (Bucks), adds interpretative comments, whereas the other, Oxford University Hospitals (Oxford), does not. We used two outcome measures: the percentage of patients with thyroid-stimulating hormone (TSH) within the reference interval on repeat testing and the timing of repeat TSH testing samples, according to the National Institute for Health and Care Excellence guidance (NG145).

Results: We identified 18 242 and 31 655 hypothyroid patients (9.0% and 7.7% of the population tested) in Bucks and Oxford, with a total of 121 961 and 247 639 tests over the evaluation period, respectively. The proportion of TSH results within the reference interval (83.4% in Bucks, 83.9% in Oxford) was similar in both Trusts, as was TSH concentration (median TSH concentration 1.60 (IQR 0.78-2.82) mU/L in Bucks, 1.68 (IQR 0.97-2.76) in Oxford). The interval between tests was shorter in Oxford, but differed significantly from NG145 in both Trusts. Differences were statistically significant for both outcome measures, but of questionable clinical significance.

Conclusions: Adding interpretative comments to results of thyroid function tests does not appear to affect the distribution of TSH concentrations in primary care patients on thyroxine replacement or the intervals between tests in a clinically meaningful way.