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

The proportion of TSH results within the reference interval was similar between the laboratory that adds interpretative comments and the one that does not.

• 83.4% of TSH results were within the reference interval at Buckinghamshire Health Trust (Bucks), which adds interpretative comments.
• 83.9% of TSH results were within the reference interval at Oxford University Hospitals (Oxford), which does not add interpretative comments.
• Differences were statistically significant but described as of 'questionable clinical significance.'
• Total tests analyzed: 121,961 at Bucks and 247,639 at Oxford over the evaluation period January 2016 to August 2023.

Median TSH concentrations were similar between the two Trusts.

• Median TSH concentration in Bucks was 1.60 mU/L (IQR 0.78–2.82).
• Median TSH concentration in Oxford was 1.68 mU/L (IQR 0.97–2.76).
• Both Trusts used the same instruments, facilitating direct comparison.
• Differences were statistically significant but of questionable clinical significance.

The interval between thyroid function tests was shorter in Oxford than in Bucks, but both Trusts differed significantly from NICE guidance (NG145).

• Both Trusts showed significant deviation from NG145-recommended testing intervals.
• The interval between tests was shorter in Oxford (which does not add interpretative comments) compared to Bucks (which does).
• Differences in testing intervals were statistically significant but of questionable clinical significance.
• NICE guidance NG145 was used as the reference standard for appropriate repeat testing timing.

The study identified large cohorts of hypothyroid patients treated in primary care at both NHS Trusts.

• 18,242 hypothyroid patients were identified at Bucks, representing 9.0% of the population tested.
• 31,655 hypothyroid patients were identified at Oxford, representing 7.7% of the population tested.
• The evaluation period spanned January 2016 to August 2023.
• Both Trusts served similar demographics and used the same analytical instruments.

Two outcome measures were used to assess the impact of interpretative comments: the percentage of TSH results within the reference interval and the timing of repeat TSH testing relative to NICE NG145 guidance.

• Outcome measure 1: percentage of patients with TSH within the reference interval on repeat testing.
• Outcome measure 2: timing of repeat TSH testing samples according to NICE NG145.
• The study design was a retrospective comparison between two NHS Trusts with differing interpretative comment policies.
• One laboratory (Bucks) adds interpretative comments to thyroid function test reports; the other (Oxford) does not.