There is a general interest to know whether lipoprotein(a) [Lp(a)] is under hormonal control. Hypothyroidism is a well known cause of secondary hyperlipidemia, which mainly affects low density lipoprotein (LDL) cholesterol levels, but the result on the effects of L-T-4 replacement therapy on the Lp(a) concentration is controversial. We studied 12 severely hypothyroid, hypercholesterolemic patients under basal conditions and during L-T-4 treatment. We found a rapid decrease in both LDL cholesterol (5.71 +/- 0.62 vs. 4.37 +/- 0.44 mmol/L basally and after 1 month of thyroid replacement, respectively) and apolipoprotein-B (Apo-B) levels (1.89 +/- 0.02 vs. 1.52 +/- 0.17 g/L, respectively); these changes persisted for up 1 yr of analytical euthyroidism and paralleled the improvement in the thyroid status of the patients. In contrast, the plasma Lp(a) concentration did not change at any time (496 +/- 123, 464 +/- 128, and 441 +/- 110 mg/L under basal conditions and after 1 and 14-15 months of thryoid replacement, respectively), and the small fluctuations observed in some patients did not correlate with those in LDL cholesterol or Apo-B, and were not associated with any particular Apo(a) phenotype. In relation to HDL fractions, high density Lipoprotein(3) (HDL(3)) remained stable, but HDL(2) cholesterol and phospholipid levels decreased during treatment, changes that were the inverse of those in postheparin plasma hepatic Lipase activity. Patients in the present study were normotriglyceridemic, except one who was hypertriglyceridemic at diagnosis, but even in this patient, triglyceride levels were unaffected by T-4 substitution therapy, as was postheparin plasma lipoprotein lipase activity. The changes observed in LDL, HDL(2), and hepatic lipase activity delineate the lipoprotein-related response to T-4 replacement therapy, whereas potential individual fluctuations in Lp(a) levels are probably more dependent on other factors, such as the production rate, which are not affected by thyroid hormones.