TSH response to TRH in euthyroid, hypercholesterolemic patients treated with graded doses of dextrothyroxine

In an attempt to determine the minimum dose of D-Thyroxine (D-T4) which will suppress pituitary TSH response to TRH, we have treated 6 euthyroid, hypercholesterolemic patients with graded doses of D-T4. TSH response was suppressed in 3 patients with 3 mg and in the remaining 3 patients with 4 mg D-T4 administered once daily. The mean TSH suppressive dose of 3.5 mg, as determined in this study, is considerably less than the 6 mg daily dose given to patients treated with D-T4 in the Coronary Drug Project. This suggests that the adverse effects observed with D-T4 treatment in the Coronary Drug Project may have been due to mild, undetected hyperthryroidism. D-T4 treatment in our patients was not associated with an increase in heart rate or ventricular ectopic beats as determined by Holter monitoring. However, bile samples obtained at the time of TSH suppression showed a significant increase in lithogenic index. In four patients, TSH suppressive doses of D-T4 were associated with a 12% decrease in mean cholesterol and a 17% decrease in mean LDL cholesterol concentrations.