Adequacy of thyroid hormone replacement in a general population

Background: Suboptimal thyroid hormone replacement may carry harmful health consequences. Aims: Our objectives were to determine the prevalence and factors associated with inadequate replacement in patients receiving treatment with levothyroxine. Design: Retrospective general practice audit. Methods: We identified levothyroxine users through electronic searches of primary care records in all 11 practices within a county borough. The adequacy of thyroid hormone replacement was determined from the current serum, serum thyrotropin (TSH) as: (i) adequate replacement (normal TSH; 0.4-4.0 mU/l); (ii) over replacement (low TSH; < 0.4 mU/l); and (iii) under replacement (high TSH; > 4.0 mU/l). Results: Out of a registered patient population of 58 567, we identified 1037 patients who were first included in the hypothyroidism disease register between January 2004 and December 2009 (mean age 62.4 +/- 15.9 years; female 85.9%, male 14.1%). Inadequate replacement was seen in 385 patients (37.2%), comprising 205 patients (19.8%) with over replacement and 180 patients (17.4%) with under replacement. Step-wise logistic regression showed that the factors associated with under replacement were male gender [odds ratio (OR) 2.85, confidence interval (CI) 1.86-4.38; P < 0.001 and younger age (OR 0.88, CI 0.80-0.98; P=0.02 per 10 year increase in age) while longer duration of treatment was associated with over-treatment (OR 1.06, CI 1.01-1.10). A thyroid function test was performed in the preceding 12 months in 914 patients (88.1%) and appropriate dose adjustments had been made in 81.0% (312/385) of patients with abnormal results. Conclusions: Despite frequent monitoring and dose adjustment activities, inadequate thyroid hormone replacement remained a problem in over a third of levothyroxine users in this population.