Effect of thyroxine therapy on serum lipoproteins in patients with mild thyroid failure

Levothyroxine (T4) therapy in individuals with mild thyroid failure may lower mean serum total and LDL cholesterol concentrations.

A systematic review including 13 studies with a total of 247 subjects was abstracted in DARE. Three studies (23%) were RCTs with 81 participants.

The mean decrease in the serum total cholesterol concentration (13 studies) was –0.20 mmol/l (–7.9 mg/dl; 95% CI: –0.09 to 0.34). The decline in serum total cholesterol was directly proportional to its baseline concentration. Studies enrolling hypothyroid participants receiving suboptimal T4 doses reported significantly larger decreases in serum total cholesterol after thyroid-stimulating hormone normalisation than those enrolling previously untreated individuals with mild thyroid failure. The authors conclude the reduction in serum total cholesterol may be larger in individuals with higher pre-treatment cholesterol levels, and in hypothyroid individuals taking suboptimal T4 doses. There seems to be no significant effects of T4 on serum HDL or triglyceride concentrations.

Sixty-six women with proven subclinical hypothyroidism (TSH, 11.7 ± 0.8 mIU/liter) were randomly assigned to receive L-thyroxine or placebo for 48 wk . In the L-thyroxine group (n = 31) total cholesterol and low density lipoprotein cholesterol were significantly reduced (–0.24 mmol/liter, 3.8%, P = 0.015) and –0.33 mmol/liter, 8.2%, P = 0.004), respectively). Low density lipoprotein cholesterol decrease was more pronounced in patients with TSH levels greater than 12 mIU/liter or elevated low density lipoprotein cholesterol levels at baseline. Two clinical scores assessing symptoms and signs of hypothyroidism (Billewicz and Zulewski scores) improved significantly (P = 0.02).

Comment: The quality of evidence is downgraded by sparse data and study limitations. Since the analysis in the systematic review compares differences between pre- and post-treatment cholesterol levels, the intervention studies may be of a before-and-after type rather than randomised controlled trials. Therefore, the decreases in cholesterol reported may have been due to factors other than the thyroid substitution therapy, e.g. dietary factors.

Ədəbiyyat

1. Danese MD, Ladenson PW, Meinert CL, Powe NR. Clinical review 115: effect of thyroxine therapy on serum lipoproteins in patients with mild thyroid failure: a quantitative review of the literature. J Clin Endocrinol Metab 2000 Sep;85(9):2993-3001.
2. Chu JW, Crapo LM. The treatment of subclinical hypothyroidism is seldom necessary. J Clin Endocrinol Metab 2001 Oct;86(10):4591-9.