Producing too many or too few thyroid hormones increases the risk of abnormal blood cholesterol.
We take a closer look at the link between thyroid and cholesterol in this article. We also discuss some of the ways to manage cholesterol and thyroid conditions.
The thyroid gland
The thyroid gland regulates body temperature, mood, and development.
The thyroid is a gland located in the neck. It produces two hormones, known as the thyroid hormones. The main ones are triiodothyronine (T3), which is the active form of thyroid hormone, and thyroxine (T4), which the body converts to T3.
Thyroid hormones play a role in regulating:
- brain development in children
- body temperature
- normal growth and development
- the function of the heart, brain, muscle, and other organs
The pituitary gland secretes thyroid-stimulating hormone (TSH) to stimulate the thyroid to produce hormones.
The thyroid gland also produces a hormone called calcitonin, which helps maintain strong bones by keeping the amount of calcium in the blood low.
When the thyroid does not produce enough thyroid hormones, it is underactive, which is called hypothyroidism. Producing too many hormones leads to an overactive thyroid, which is called hyperthyroidism.
Approximately 4.6 percent of people aged 12 or older in the United States have an underactive thyroid.
An underactive thyroid causes the following symptoms:
- dry skin and hair
- fertility issues
- heavy or irregular periods
- increased sensitivity to cold temperatures
- low heart rate
- pain in the muscles and joints
- reduced sweating
- weight gain
An underactive thyroid often arises from an autoimmune disease, such as Hashimoto's thyroiditis, or surgical removal of the thyroid.
Less commonly, the thyroid gland goes into overdrive and produces too many thyroid hormones. Approximately 1.2 percent of people in the U.S. have an overactive thyroid.
Symptoms of an overactive thyroid include:
- diarrhea or frequent bowel movements
- difficulty sleeping
- increased sensitivity to heat
- mood swings
- muscle weakness
- nervousness or irritability
- rapid heartbeat
- shaky hands
- weight loss
Several conditions and situations can cause an overactive thyroid, including an autoimmune condition called Graves' disease, inflammation of the thyroid (thyroiditis), and taking too much iodine or thyroid hormone pills.
A cholesterol buildup in the arteries can cause heart disease.
Cholesterol is a waxy substance that is present in every cell in the body. The body uses cholesterol to make hormones as well as bile acids to help digest fat.
When too much cholesterol builds up in the arteries, it can cause problems, such as heart disease.
Cholesterol travels through the blood by attaching itself to a protein. This bundle of cholesterol and protein is called a lipoprotein.
The main lipoprotein bundles are:
- High-density lipoprotein (HDL): HDL is so-called because there is a high cholesterol to protein ratio. It is also known as "good" cholesterol because it helps the body get rid of cholesterol from the blood.
- Low-density lipoprotein (LDL): LDL features a low cholesterol to protein ratio. It is also known as "bad" cholesterol. Higher LDL rates indicate a higher risk of heart disease.
Although dietary causes (in those predisposed to it) may cause a buildup of bad cholesterol in the blood, it may also arise from some medical conditions, including thyroid disorders.
Understanding the link between the thyroid and cholesterol
The link between the thyroid and cholesterol can result in conditions affecting each of them:
Can an underactive thyroid cause high cholesterol?
Thyroid hormones, especially T3, play an essential role in helping the liver process and remove any excess cholesterol from the body.
When the body does not produce enough thyroid hormones, the liver cannot process as much cholesterol as it should. An underactive thyroid means that the body removes less "bad" cholesterol from the blood than it should, which often leads to high levels of "bad" and total cholesterol.
According to research, up to 13 percent of people with high blood cholesterol also have an underactive thyroid.
The same research notes that societies, such as the American Association of Clinical Endocrinologists and the American Thyroid Association, recommend that people newly-diagnosed with high cholesterol get tested for an underactive thyroid.
The research also suggests that a person's cholesterol levels may improve once they have received treatment for an underactive thyroid, even if they do not require cholesterol-lowering medications.
Even mildly low thyroid hormone levels (subclinical hypothyroidism) can lead to high cholesterol. According to 2012 research, elevated TSH levels can also cause high cholesterol, even if T3 and T4 are not elevated.
Can an overactive thyroid cause low cholesterol?
An overactive thyroid might have the opposite effect and cause low levels of cholesterol in the blood. However, there is no scientific proof that low cholesterol increases the risk for health problems.
Testing thyroid and cholesterol levels
A blood sample will check levels of TSH, thyroid hormones, and cholesterol.
People with symptoms of an underactive or overactive thyroid should see a doctor for further testing, especially if their cholesterol levels are high or low.
A doctor will take a sample of blood to check for levels of TSH and thyroid hormones. The results will show if the thyroid is overactive, underactive, or is functioning normally.
The doctor or lab technician should be able to test blood cholesterol levels from the same blood sample, but for accurate results, people must fast for 8 to 12 hours beforehand.
Treating thyroid problems and high cholesterol
If the thyroid is underactive or overactive, a doctor can recommend a treatment plan to manage the thyroid condition and abnormal cholesterol levels.
People with an underactive thyroid will often see improvements in cholesterol levels if they take a T4-replacement medication such as:
- Levothyroxine sodium
Where thyroid hormone levels are only slightly below normal, the doctor may recommend a statin or other treatment for cholesterol instead of a thyroid-replacement drug.
Commonly prescribed statins include:
- Crestor (rosuvastatin calcium)
- Lescol (fluvastatin)
- Lipitor (atorvastatin)
- Mevacor, Altoprev (lovastatin)
- Pravachol (pravastatin)
- Zocor (simvastatin)
Other treatments for high cholesterol include:
- weight loss for those who are overweight
- regular exercise
- dietary changes, including reducing saturated fat and increasing fiber intake
People with an overactive thyroid can get relief from their symptoms by taking medications to reduce thyroid hormone production. Lower levels of the hormone may also increase their cholesterol levels if they are low.
If medications do not work, the doctor may suggest radioactive iodine to shrink the thyroid gland or surgery to remove part of the gland, which will reduce hormone levels. People who have low cholesterol caused by high thyroid hormone levels may see their cholesterol levels increase as a result of the treatment.
There is a clear link between thyroid disorders and abnormal cholesterol levels. Those who are newly-diagnosed with high cholesterol should ask their doctor to check their thyroid hormone levels.
Similarly, those who have an underactive thyroid should regularly monitor their blood cholesterol levels and take steps to reduce the risk of developing high cholesterol.
Some people may see results once they treat their thyroid disorder. However, others with high cholesterol levels will need to take cholesterol-lowering medications and make lifestyle changes to bring cholesterol back into normal levels.