BACKGROUND
Cholesterol and lipoproteins have been shown to be associated with heart disease. LDL-cholesterol is known as the bad cholesterol and increased levels are associated with an increase in heart disease. Lipoaprotein (a) (Lp(a)) is also associated with increased heart disease. HDL-cholesterol is known as the good cholesterol as increased levels seem to lower the risk of heart disease.

Thyroid hormone has a direct effect on cholesterol levels. Hypothyroid patients have increased cholesterol levels compared to individuals with normal thyroid function. Treatment with thyroid hormone often lowers the cholesterol levels in patients with hypothyroidism. In hyperthyroid patients, on the other hand, cholesterol levels are low and correction of the hyperthyroid state with medications may cause an increase in cholesterol levels. This study investigates the effect of thyroid hormone replacement therapy versus observation for overt and subclinical hypothyroidism and hyperthyroidism on various cholesterol and lipoprotein levels.

THE FULL ARTICLE TITLE
Kowal A et al 2020 Treatment of thyroid dysfunction and serum lipids: A systematic review and meta-analysis. J Clin Endocrinol Metab 105:dgaa672. PMID: 32954428.

SUMMARY OF THE STUDY
The authors looked at many published articles that studied patients who received thyroid hormone therapy for overt or subclinical hypothyroidism and patients with hyperthyroidism who received treatment for hyperthyroidism. They choose 166 studies that included 12,855 patients. Regarding the studies on hyperthyroid patients: the average age of the patients was 48 years and 20% were males.

Treatment of patients with overt hyperthyroidism (low TSH and high FT₄ and T₃) was associated with an increase of total cholesterol levels of about 44 mg/ dL. LDL-cholesterol increased by 31 mg/dL and Lp(a) increased by 4 mg/dL. Patients treated for subclinical hyperthyroidism (low TSH and normal FT₄ and T₃) did not have changes in their lipid levels.

In the studies looking at overt hypothyroidism, the average age was also 48 years. When patients with overt hypothyroidism (high TSH and low FT₄) were treated with levothyroxine their total cholesterol levels went down by 58 mg/dl, LDL-cholesterol was lowered by 41 mg/dL, HDL-cholesterol was lowered by 4 mg/DL, triglycerides by 7 mg/dL and Lp(a) by 5 mg/dL. For patients with subclinical hypothyroidism (high TSH and normal FT₄ ), treatment with levothyroxine, showed a small decrease of total cholesterol levels by 12 mg/dL, LDL-cholesterol by 11 mg/dL and triglycerides by 4 mg/dL.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study confirms that treatment of hyper and hypothyroidism is associated with changes in cholesterol and lipoprotein levels. Treatment of overt hypothyroidism with levothyroxine significantly lowers cholesterol levels while treatment of hyperthyroidism with antithyroid medications causes an increase in total cholesterol, LDLcholesterol and HDL-cholesterol. The effects of treating patients with subclinical thyroid disease resulted in much smaller changes. While overt thyroid disease requires therapy, treatment of subclinical thyroid disease is much less straightforward. One drawback of these studies is that none of the studies included looked at the risk of heart disease. However, this paper suggests that the small improvement in cholesterol after treating subclinical hypothyroidism may be of benefit for some patients.

— Susana Ebner MD

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