PT  - JOURNAL ARTICLE
AU  - Symonds, Christopher
AU  - Kline, Gregory
AU  - Gjata, Inelda
AU  - Rose, Marianne Sarah
AU  - Guo, Maggie
AU  - Cooke, Lara
AU  - Naugler, Christopher
TI  - Levothyroxine prescribing and laboratory test use after a minor change in reference range for thyroid-stimulating hormone
AID  - 10.1503/cmaj.191663
DP  - 2020 May 04
TA  - Canadian Medical Association Journal
PG  - E469--E475
VI  - 192
IP  - 18
4099  - http://www.cmaj.ca/content/192/18/E469.short
4100  - http://www.cmaj.ca/content/192/18/E469.full
SO  - CMAJ2020 May 04; 192
AB  - BACKGROUND: Prescribing of levothyroxine and rates of thyroid function testing may be sensitive to minor changes in the upper limit of the reference range for thyroid-stimulating hormone (TSH) that increase the proportion of abnormal results. We evaluated the population-level change in levothyroxine prescribing and TSH testing after a minor planned decrease in the upper limit of the reference range for TSH in a large urban centre with a single medical laboratory.METHODS: Using provincial administrative data, we compared predicted volumes of TSH tests with actual TSH test volumes before and after a planned change in the TSH reference range. We also determined the number of new levothyroxine prescriptions for previously untreated patients and the rate of changes to the prescribed dose for those on previously stable, long-term levothyroxine therapy before and after the change in the TSH reference range.RESULTS: Before the change in the TSH reference range, actual and predicted monthly volumes of TSH testing followed an identical course. After the change, actual test volumes exceeded predicted test volumes by 7.3% (95% confidence interval [CI] 5.3%–9.3%) or about 3000 to 5000 extra tests per month. The proportion of patients with newly “abnormal” TSH results almost tripled, from 3.3% (95% CI 3.2%–3.4%) to 9.1% (95% CI 9.0%–9.2%). The rate of new levothyroxine prescriptions increased from 3.24 (95% CI 3.15–3.33) per 1000 population in 2013 to 4.06 (95% CI 3.96–4.15) per 1000 population in 2014. Among patients with preexisting stable levothyroxine therapy, there was a significant increase in the number of dose escalations (p < 0.001) and a total increase of 500 new prescriptions per month.INTERPRETATION: Our findings suggest that clinicians may have responded to mildly elevated TSH results with new or increased levothyroxine prescriptions and more TSH testing. Knowledge translation efforts may be useful to accompany minor changes in reference ranges.See related article at www.cmaj.ca/lookup/doi/10.1503/cmaj.200511