Context

Current guidelines recommend TSH for initial biochemical evaluation of thyroid function, with borderline TSH abnormalities a common finding. The likelihood of a borderline TSH progressing to overt abnormality is not well characterised at the population level.

Objective

To determine risk factors and likelihood for progression of borderline TSH to overt abnormality.

Design, Setting, Participants, & Intervention

Population-based retrospective longitudinal data-linkage study for TSH tests performed in Tasmania (1996-2013). Kaplan-Meier methodology was used to summarise conversion time for overt TSH elevation (≥10mU/L) and overt suppression (≤0.1mU/L) in patients whose initial TSH was borderline elevated (BeTSH) (4.0-9.99mU/L) and borderline suppressed (BsTSH) (0.10-0.39mU/L) respectively.

Main Outcome Measures

Progression from borderline to overt TSH abnormality.

Results

A total of 1,296,060 TSH tests were performed on 367,917 patients. Of these, 14,507 (3.9%) patients had BeTSH on initial assessment; mean age 51.4±21.8 years and median TSH of 5.0mU/L (IQR 4.4, 5.2). Patients aged ≥80 years were most likely to progress (HR=2.09 compared to age ≤20 years reference group [95% CI: 1.64, 2.68]). Patients aged 20-39 years had the second highest rate of progression (HR=1.49 [95% CI: 1.18, 1.88]). 7,883 (2.14%) patients had BsTSH; mean age 50.7±22.1 years and median TSH 0.30mU/L (IQR 0.22, 0.35). Patients aged 60-79 years had the highest rate of progression to overt TSH suppression (HR = 2.47 compared to age ≤20 years reference group [95% CI: 1.88, 3.22]).

Conclusions

Follow-up intervals for patients with borderline TSH abnormalities should take into account patient age as a progression risk factor.

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