Current guidelines recommend TSH testing for initial evaluation of thyroid function. Borderline TSH abnormalities (0.1-0.4mIU/L and 4.0-10mIU/L) are seen in this context. However, unlike subclinical hypothyroidism and hyperthyroidism, the natural history of borderline TSH abnormalities, including frequency of progression to overtly abnormal TSH, is poorly characterised.
To determine the likelihood of progression to overt TSH abnormality for patients with borderline TSH abnormalities.
We performed a population-based retrospective longitudinal data-linkage study for TSH tests performed in Tasmania from 1996-2013. TSH results were linked to identifier numbers to permit patient-specific follow-up. Kaplan-Meier methodology was used to summarise time to conversion to overtly elevated (>10mIU/L) and suppressed (<0.1mIU/L) TSH for patients with borderline elevated and suppressed TSH, respectively.
367,917 patients had 1,296,060 TSH tests analysed. Of the 21,426 patients with borderline-elevated TSH, those in the >80yo age group at the time of incident testing were most likely to progress to full elevation (conversion rate 9.0% at 2.5yrs, 11.3% at 5yrs, 12.6% at 10yrs), with those in the <20yo age group least likely to progress (3.8% at 2.5yrs, 5.1% at 5yrs, 6.5% at 10yrs).
Of the 20,366 patients found to have borderline-suppressed TSH, those in the 60-79yo age group at the time of incident testing were most likely to progress to full suppression (12.1% at 2.5yrs, 16.0% at 5yrs, and 19.4% at 10yrs), with those in the <20yo group again least likely to progress (5.3% at 2.5yrs, 6.7% at 5yrs, 9.0% at 10yrs).
Periodic TSH re-testing is appropriate for patients with borderline TSH abnormalities, but the re-testing interval should be determined by the likely rate of progression. Progression from borderline to overt TSH abnormality increases with the patient's age at the time of initial testing. Borderline TSH suppression is also more likely to progress than borderline TSH elevation.