Context: Subclinical hypothyroidism is a common clinical problem defined as an elevation in serum thyrotrophin (TSH) with normal circulating free thyroid hormone levels. Empiric treatment of mild subclinical hypothyroidism (mSCH) is controversial.
Objective: To evaluate the change in the levothyroxine prescribing rate and TSH testing following a decrease in the TSH reference range from 6 to 4 mU/L in a large urban center.
Design, Setting, Patients, Outcome Measures: With 45,00065,000 TSH tests performed per month, we were able to build a very robust model to accurately predict the volume of TSH tests and compare this to actual TSH test volumes before and after the reference range change. We also evaluated the dispensation rate of new levothyroxine prescriptions and new levothyroxine dosage increases before and after the change.
Results: Prior to the TSH reference range change, the actual and predicted TSH volumes per month followed an almost superimposable pattern. After the change, a persistent separation emerges with actual TSH test volumes exceeding those predicted by 7.3%. Patients labelled with mSCH almost tripled, from 3.3% to 9.1%. New levothyroxine prescriptions increased by 25.3% from 2013 to 2014 (P<0.001). For pre-existing levothyroxine users, there was a significant increase in dose escalation (P<0.001).
Conclusions: Clinicians may rely heavily on TSH to make decisions about levothyroxine prescribing and dosage change even with only modestly elevated results. We speculate there may be a knowledge care gap regarding the lack of strong evidence supporting the treatment of mSCH.