Lipoprotein(a) [Lp(a)] is a low-density lipoprotein (LDL) particle in which apolipoprotein B-100 (apoB) is attached to a glycoprotein called apolipoprotein(a) [apo(a)]. Apo(a) has several genetically determined phenotypes differing in molecular weight, to which Lp(a) concentrations in plasma are inversely correlated. High plasma levels of Lp(a) are associated with atherosclerotic diseases. It is therefore of interest to study whether factors other than the apo(a) gene locus are involved in the regulation of Lp(a) concentrations. We measured plasma concentrations of Lp(a) and other lipoproteins and determined apo(a) phenotypes in 31 patients with hyperthyroidism, before and after the patients had become euthyroid by treatment. The mean concentration of LDL cholesterol rose from 2.67 to 3.88 mmol/l (P < 0.01), apoB rose from 0.79 to 1.03 g/l (P < 0.01), and the median Lp(a) concentration increased from 9.74 to 18.97 mg/dl (P < 0.01) on treatment. Lp(a) concentrations were inversely associated to the size of the apo(a) molecule both before (P < 0.01) and after treatment (P < 0.01). The increase in Lp(a) was significant in patients with high molecular weight apo(a) phenotypes (n = 9; P < 0.01) and in patients with low molecular weight apo(a) phenotypes (n = 16; P < 0.01), but not in those with apo(a) "null types" (n = 6; P = 0.5). The low levels LDL cholesterol and apoB in untreated hyperthyroidism may result from increased LDL receptor activity. The increase in Lp(a) levels were not correlated with the increase in LDL cholesterol or apoB.(ABSTRACT TRUNCATED AT 250 WORDS)