MON-601 Optimizing the Mean TSH Target Range for Thyroid Hormone (L-T4) Replacement in Primary Hypothyroidism

A commonly advocated TSH target range for guiding the dosage of thyroid hormone (L-T4) replacement in primary hypothyroidism is 0.3 to 3.0 mU/L. This is based on the physiological acceptability of individual TSH levels within this range. Individual serum TSH concentrations, however, vary about their mean TSH concentration even in patients who have been stable on the same dose of L-T4. Predictably, when the mean TSH level is near the upper (or lower limit) of the above stated target range, some individual TSH levels will be above (or below) the target range. This study was designed to assess the variability of individual TSH levels above and below their corresponding mean serum level, and to identify a range for mean TSH levels that would minimize the frequency of the associated individual TSH levels being outside of the 0.3 to 3.0 mU/L target range. Sixty patients (yielding 368 TSH determinations) with primary hypothyroidism were receiving L-T4 replacement in an appropriate and unchanged dosage that maintained their mean TSH levels between 0.3 to 3.0 mU/L for up to 3 years. Achievement of the stable L-T-4 dosage was declared after an initial titration stage before the beginning of the stable interval. Patients non-compliant with their prescribed L-T4 dosage were excluded. The number of TSH determinations and the percentages of the patients whose mean TSH levels were between the bracketed subsets of the 0.3 to 3.0 mU/L target range (below), are presented as having individual TSH levels >3.0 mU/L, 0.3-3.0 mU/L, and <0.3 mU/L respectively: [0.3 to <0.5] 13, 0.0%, 61.5%, 38.5%; [0.5 to <1.0] 39, 0.0%, 82.1%, 17.9%; [1.0 to <1.5] 90 3.3%, 92.2%, 4.4%; [1.5 to <2.0] 141, 5.7%, 93.6%, 0.7%; [2.0 to <2.5] 59, 15.3%, 84.7%, 0.0%; [2.5 to 3.0] 26, 34.6%, 65.4%, 0.0%. Based on this data, establishing a mean TSH level of 1.0 to 2.0 mU/L would minimize the frequency of individual TSH levels above or below the physiologic TSH target range of 0.3 to 3.0 mU/L and, in doing so, minimize the frequency of rechecking serum TSH levels or unnecessarily changing dosages of L-T4.