Circulating levels of oxidized low-density lipoprotein in overt and mild hypothyroidism

The increased low-density lipoprotein cholesterol (LDL-C) levels in hypothyroidism may enhance the formation of oxidized LDL (oxi-LDL) that may consequently generate foam cells by their uptake by the macrophages. The goal of this study was to investigate whether plasma circulating oxi-LDL levels are elevated in mild and in overt hypothyroidism, if the concentration of oxi-LDL is influenced in a short-term treatment period by thyroid hormone, and whether correlations exist between serum concentration of thyrotropin (TSH), thyroid hormone, and cholesterol. Thirty-nine patients with overt hypothyroidism (OH), 41 patients with mild thyroid failure (MTF), and 57 controls (CNTR) were investigated. Serum TSH concentrations were increased in OH (18 +/- 6 mU/L) and in MTF (6 +/- 2 mU/L), whereas in CNTR the levels were 1.6 +/- 0.3 mU/L. Plasma circulating levels of oxi-LDL were measured by a new enzyme-linked immunosorbent assay (ELISA) kit (normal range, 40-75 mU/L) and they were found statistically significantly increased in OH compared to MTF (86 +/- 16 mU/L vs. 73 +/- 13 mU/L; p < 0.01) and to CNTR (62 +/- 11 mU/L; p < 0.001). Smokers in all groups exhibited statistically significant higher plasma oxi-LDL levels compared to nonsmokers. The percentage of increase amounted to 17.7% in OH, to 9.8% in MTF, and to 8% in CNTR. Replacement treatment with levothyroxine over a period of 3 months in 12 of 39 patients with OH and in 14 of 41 patients with MTF resulted in a statistically significant decrease of oxi-LDL only in the OH group. Thus, plasma oxi-LDL decreased in OH from 82 +/- 12 mU/L to 73 +/- 10 mU/L (p < 0.05), to the upper normal level, and in MTF from 68 +/- 5 mU/L to 64 +/- 5 mU/L, respectively. In conclusion, we can state that circulating oxi-LDL levels are elevated in untreated overt hypothyroidism, they tend to be higher in mild thyroid failure, they are severely affected by smoking, however, they need a longer time course to decrease via thyroxine treatment.