From the Guidelines
The thyroid gland significantly affects cholesterol levels, as thyroid hormones regulate metabolism and lipid processing, with hypothyroidism leading to elevated cholesterol levels and hyperthyroidism causing abnormally low cholesterol levels. When thyroid function is low (hypothyroidism), the body's ability to clear cholesterol from the bloodstream decreases, leading to elevated total cholesterol, LDL (bad) cholesterol, and triglyceride levels [ 1 ]. This occurs because thyroid hormones normally stimulate LDL receptors in the liver that remove cholesterol from circulation, and they also enhance the activity of enzymes involved in cholesterol metabolism. Additionally, thyroid hormones increase the body's overall metabolic rate, which affects how quickly fats are used for energy.
Some key points to consider:
- Thyroid hormones have been found to affect plasma Lp(a) levels, with hypothyroid patients having significantly higher and hyperthyroid patients having lower Lp(a) concentrations [ 1 ].
- Appropriate treatment of thyroid dysfunction can lead to changes in Lp(a) levels towards those of euthyroid controls [ 1 ].
- While some studies suggest that thyroid hormone therapy may reduce total and LDL cholesterol levels in individuals with subclinical hypothyroidism [ 1 ], this finding has not been consistently confirmed in randomized controlled trials.
In hypothyroidism, treating the underlying thyroid condition with levothyroxine (typically starting at 25-50 mcg daily, gradually increasing as needed based on thyroid function tests) often normalizes cholesterol levels without requiring separate cholesterol-lowering medications. Conversely, hyperthyroidism (overactive thyroid) can cause abnormally low cholesterol levels due to increased metabolism. Regular thyroid function testing (TSH, free T4) is important for anyone with unexplained cholesterol abnormalities, as addressing thyroid dysfunction may be the primary treatment needed [ 1 ].
From the Research
Thyroid Gland and Cholesterol Levels
The thyroid gland plays a significant role in regulating cholesterol levels in the body.
- Hypothyroidism is closely associated with increased serum total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and triglyceride (TG) 2.
- Thyroid hormones (THs) modulate cholesterol production, transformation, and clearance, and changes in THs can lead to hyperlipidemia 2, 3.
- The pathologic development of hypothyroidism-related hyperlipidemia is associated with down-regulated THs and up-regulated thyroid-stimulating hormone (TSH) in serum 2.
Mechanisms of Thyroid Hormone Action
Thyroid hormones regulate a variety of pathways involved in the metabolism of carbohydrates, lipids, and proteins in several target tissues.
- Hyperthyroidism induces a hyper-metabolic state characterized by increased resting energy expenditure, reduced cholesterol levels, increased lipolysis, and gluconeogenesis followed by weight loss 4.
- Hypothyroidism induces a hypo-metabolic state characterized by reduced energy expenditure, increased cholesterol levels, reduced lipolysis, and gluconeogenesis followed by weight gain 4.
- The intracellular availability of thyroid hormone is regulated by a mechanism of activation/inactivation catalyzed by three seleno-proteins: type 1 and type 2 iodothyronine deiodinase (D1 and D2) and type 3 iodothyronine deiodinase (D3) 4.
Thyroid Disorders and Lipid Levels
Thyroid disorders, including overt and subclinical hypothyroidism, considerably alter lipid profile and promote cardiovascular disease.
- High thyroid-stimulating hormone (TSH) is associated with a non-favorable lipid profile, although TSH has no cutoff threshold for its association with lipids 5.
- Thyromimetics represent a new class of hypolipidemic drugs that can improve the lipid profile, potentially accelerate energy expenditure, and lessen the risk of cardiovascular disease 5.
- Hypothyroidism also contributes to the production of dysfunctional high-density lipoprotein (HDL) particles, which can further increase the risk of cardiovascular disease 2.