[Individual calculation of the minimal effective levothyroxine dose in prolonged suppression tests]

A formula for calculating the minimum suppressive levothyroxine dose in prolonged suppression tests using body weight and TSH level is derived from a large number of cases.

In 1239 euthyroid patients (TSH > 0.30 mU/l) a suppression test with levothyroxine substitution for at least 6 weeks was performed. The hormone dose was primarily based on the initial TSH level using the empirical value TSH x 100, with a range between 50 and 150 micrograms per day.

In 900 patients (73%), the TSH level was suppressed during the medication (TSH < 0.30 mU/l). Among them, we found 211 cases with an intermediate TSH suppression (TSH between 0.10 and 0.20 mU/l). Following the formula f = levothyroxine dose [microgram]/(body weight [kg] x initial TSH level [mU/l]) an average factor of 1.57 was calculated to obtain the suppressive dose in this group. As usually a complete TSH suppression (< 0.10 mU/l) is required, we recommend factor 2 for practical use. A consideration of the body weight revealed that 313 patients actually received a hormone dose equivalent to 2 x weight x TSH. In this group, a TSH suppression was found in 262 patients (84%), a factitial hyperthyreosis was not observed. When contemplating the three subgroups of this collective, who received daily doses of 50, 75 and 100 micrograms levothyroxine, respectively, we neither found a significant difference in the average body weight, nor in the value of the suppressed TSH. However, the correlation between the average initial TSH levels in these groups and the respective suppressive hormone doses clearly demonstrates their dependence on the thyroid regulation.

The minimum suppressive levothyroxine dose does not only depend on the body weight, but also on the initial TSH level. It can be estimated using the formula 2 x body weight x initial TSH (range between 50 and 150 micrograms per day).