LIPOPROTEIN AND APOLIPOPROTEIN LEVELS IN SUBCLINICAL HYPOTHYROIDISM - EFFECT OF LEVOTHYROXINE THERAPY

To assess whether subclinical hypothyroidism is associated with changes in lipoprotein fractions, 13 patients maintained in a stable state of subclinical hypothyroidism for at least 3 months were studied prior to and 2 and 4 months following restoration of a euthyroid state with incremental levothyroxine sodium therapy. Thyrotropin levels (± SEM) had decreased from 16.6 ± 3.2 mU/L to 3.1 ± 0.7 mU/L and 3.2 ± 0.7 mU/L at 2 months and 4 months. At 2 months, levothyroxine treatment led to a decrease in levels of total cholesterol from 5.5 ± 0.3 mmol/L (213 ± 12 mg/dL) to 4.8 ± 0.3 mmol/L (186 ± 12 mg/dL), in low-density lipoprotein cholesterol (LDL-C) from 3.7 ± 0.3 mmol/L (143 ± 12 mg/dL) to 2.9 ± 0.3 mmol/L (112 ± 12 mg/dL), and in apolipoprotein B from 91 ± 8 mg/dL to 74 ± 7 mg/dL. At 4 months, levels of LDL-C and apolipoprotein B remained significantly lower than pretreatment values (2.9 ± 0.2 mmol/L [112 ± 8 mg/dL] and 75 ± 6 mg/dL, respectively). While high-density lipoprotein cholesterol (HDL-C), HDL3-C, and apolipoprotein A-I were not significantly affected by levothyroxine therapy, there was a slight trend of increase in HDL2-C during levothyroxine substitution. There was also a tendency for a decrease in triglyceride levels from 1.3 ± 0.2 mmol/L (115 ± 18 mg/dL) to 0.9 ± 0.1 mmol/L (80 ± 9 mg/dL) at 4 months of levothyroxine therapy. Levels of HDL-C tended to decrease from 4.8 ± 0.4 mmol/L (186 ± 15 mg/dL) to 4.5 ± 0.5 mmol/L (174 ± 19 mg/dL) at 2 months and to 3.9 ± 0.4 mmol/L (151 ± 15 mg/dL) at 4 months. The LDL-C/HDL-C ratio is also decreased from 3.3 ± 0.3 mmol/L (128 ± 12 mg/dL) to 2.9 ± 0.5 mmol/L (112 ± 19 mg/dL) and 2.5 ± 0.3 mmol/L (97 ± 12 mg/dL) at 2 months and 4 months, respectively. These results suggest that long-term levothyroxine therapy in patients with subclinical hypothyroidism is associated with a decrease in LDL-C and apolipoprotein B levels that are reflected in a trend of decreases in cholesterol/HDL-C and LDL-C/HDL-C ratios known to have a relationship with coronary artery disease.