A study of L-thyroxine replacement

We studied consecutive requests for in vitro thyroid function tests on patients receiving L-thyroxine (T4) for primary hypothyroidism for a three month period May to July 1990, and waited a further three months before ascertaining any changes made to the daily dose of T4. Serum sensitive TSH (sTSH) concentrations were normalised in only 43% of patients treated by a hospital general physician, 50% of patients treated by a specialist endocrinologist and 52% of patients treated by their primary care physician. The respective figures for elevated serum sTSH concentrations were 43%, 33% and 32% and for suppressed levels 15%, 17% and 15%. The dose of T4 required to normalise serum sTSH levels was approximately 110 micrograms/d. In 36% of those patients with suppressed serum sTSH levels the daily dose of thyroxine was reduced. This achieved a mean reduction in the daily dose of 20 micrograms. In 30% of those with raised serum sTSH levels the daily dose of thyroxine was increased. This achieved a mean increase in the daily dose of 15 micrograms. Patients receiving T4 may have raised serum free T4 (fT4) despite normal sTSH levels indicating that serum fT4 measurement is not the test of choice for screening for adequacy of T4 replacement.