Does hypothyroidism (underactive thyroid) affect lipid profiles?

From the Guidelines

Yes, hypothyroidism significantly affects lipid metabolism, leading to elevated total cholesterol, LDL cholesterol, and triglyceride levels, along with decreased HDL cholesterol, as thyroid hormones play a crucial role in regulating lipid production, absorption, and metabolism. This occurs because when thyroid hormone levels are low, there is reduced expression of LDL receptors in the liver, decreased activity of cholesterol-clearing enzymes like lipoprotein lipase, and overall slower metabolism of lipids, as noted in studies such as 1. These changes lead to the characteristic dyslipidemia seen in hypothyroid patients.

Key Points to Consider:

• Hypothyroidism affects lipid levels by altering the expression of LDL receptors and the activity of cholesterol-clearing enzymes.
• Treatment with levothyroxine can normalize lipid profiles as thyroid function improves, which is important for reducing cardiovascular risk in affected patients, as suggested by studies like 1 and 1.
• The effect of treatment of subclinical hypothyroidism on lipid levels is mixed, with some trials suggesting potential beneficial effects, but results are inconsistent and of uncertain clinical importance, as discussed in 1.

Management Considerations:

• Treatment with levothyroxine should be considered to normalize lipid profiles and improve thyroid function, typically starting at 25-50 mcg daily for older adults or those with cardiac issues, or 75-100 mcg daily for others, adjusted every 6-8 weeks based on TSH levels.
• Clinical context is important, and the decision to initiate treatment should consider the serum TSH concentration and individual patient factors, as noted in 1.

Evidence Base:

The evidence from studies such as 1, 1, and 1 supports the impact of hypothyroidism on lipid metabolism and the potential benefits of treatment with levothyroxine on lipid profiles. However, the mixed results regarding the effect of treatment on lipid levels, as discussed in 1, highlight the need for careful consideration of individual patient factors and clinical context in management decisions.

From the FDA Drug Label

Overtreatment or undertreatment with levothyroxine sodium tablets may have negative effects on ... glucose and lipid metabolism in adult or pediatric patients

• Hypothyroidism treatment with levothyroxine sodium tablets may affect lipid metabolism.
• The FDA drug label warns that overtreatment or undertreatment may have negative effects on lipid metabolism.
• Therefore, hypothyroidism may indirectly affect lipid levels through the treatment with levothyroxine sodium tablets 2

From the Research

Effect of Hypothyroidism on Lipid Profile

• Hypothyroidism is associated with increased levels of total cholesterol, low-density lipoprotein (LDL) cholesterol, apolipoprotein B, lipoprotein(a), and possibly triglycerides 3, 4, 5.
• The degree of lipid derangement is generally proportional to the degree of TSH elevation 6.
• Thyroid hormone treatment is efficacious in reversing the lipid abnormalities in both subclinical and overt hypothyroidism 3, 6.

Lipid Alterations in Subclinical Hypothyroidism

• The effects of subclinical hypothyroidism on serum lipid values are less clear, with some studies demonstrating elevated levels of total cholesterol and LDL-C, while others have not shown any effect 4, 5.
• Apolipoprotein B levels may be increased in patients with subclinical hypothyroidism 4.
• Serum triglycerides, lipid subparticle size, and LDL-C oxidizability may be altered in subclinical hypothyroidism, but these studies have also been inconsistent 4.

Treatment with Levothyroxine

• Levothyroxine therapy has a positive impact on the lipid profile of hypothyroid patients, leading to a significant decrease in total cholesterol, LDL, and triglyceride values 3.
• However, clinical trials to date have not consistently shown a beneficial effect of levothyroxine treatment on serum lipid levels in subclinically hypothyroid patients 4, 7.
• Patients with high pre-treatment total cholesterol levels may show a significant reduction in both total cholesterol and LDL-C levels after levothyroxine treatment 7.