The Link Between Thyroid Disease and Cholesterol

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The link between thyroid disease and high cholesterol is well established. Anyone who has been diagnosed with lipid problems should have their thyroid checked, as treatment for a thyroid issue may be the key to cholesterol management. What's less well-known is that cholesterol drugs may affect thyroid disease as well. While some negatively affect the absorption of hypothyroidism drugs, one category of lipid-lowering medications (statins) may reduce the risk of thyroid eye disease in people with Graves' disease/hyperthyroidism.

The Power of the Thyroid Gland

The thyroid gland can be thought of as a master regulator of the body. Among the roles thyroid hormones play, they modulate the metabolism of lipids (such as cholesterol) by stimulating their mobilization and breakdown, and aid in the synthesis of fatty acids in the liver.

Hypothyroidism (underactive thyroid) and hyperthyroidism (overactive thyroid), therefore, can have unique effects on your cholesterol profile. For some people, these managing these effects may be the key to getting abnormal lipids under control.

Hypothyroidism and Lipids

People with hypothyroidism who have a TSH between 5 mIU/L and 10 mIU/l have a significantly higher total cholesterol and LDL level than those who do not have thyroid disease. Lipid abnormalities are usually worse with primary hypothyroidism (hypothyroidism due to lack of function of the thyroid gland) than with secondary hypothyroidism (hypothyroidism due to a lack of pituitary hormones that stimulate the thyroid gland).

The lipid abnormalities seen with hypothyroidism are thought to be primarily related to a reduction in LDL receptor activity, with other effects related to lipid metabolism as well.

Thyroid replacement with levothyroxine is usually used to treat hypothyroidism (or simply a low level of thyroid hormone) and may, for some people, be all that is needed to get cholesterol levels in check.

According to a 2017 study, even subclinical (borderline) hypothyroidism is associated with lipid abnormalities, and treatment with levothyroxine has clear benefits on both LDL and total cholesterol levels in these cases as well.

Hyperthyroidism and Lipids

While, in general, it's better to have lower LDL and total cholesterol levels, there are times when low cholesterol may not be good. An unexpected improvement in these measures may actually herald the onset of hyperthyroidism—a condition involving excess levels of thyroid hormone. If undetected and untreated, this thyroid disease can lead to complications such as eye disease, osteoporosis, atrial fibrillation (and potentially strokes or heart failure), and thyroid storm.

Hyperthyroidism is treated by reducing the level of thyroid hormone in the body, whether by medications, radioactive ablation of the thyroid gland, or surgery.

Interestingly, while hyperthyroidism may lead to low cholesterol levels, high cholesterol appears to be a risk factor for the development of Graves' disease—a particular type of hyperthyroidism.

Complications

Abnormal lipids due to hypothyroidism can increase the risk of heart disease (atherosclerosis, coronary artery disease, stroke, peripheral artery disease) and potentially, mortality. Even subclinical or borderline hypothyroidism is associated with an elevated risk of heart disease as well as the risk of death from all causes combined.

While the combination of hypothyroidism and elevated cholesterol is clearly linked with heart disease, a 2018 study found that subclinical hypothyroidism alone is associated with an increased risk of cardiovascular disease as well as all-cause mortality.

Some conditions associated with untreated subclinical hypothyroidism include coronary artery disease, cerebrovascular disease (such as strokes), peripheral artery disease, dilated cardiomyopathy, heart failure, atrial fibrillation, blood clots (deep venous thrombosis and pulmonary emboli), and chronic kidney disease.

The Effect of Hypothyroidism Treatment

In a large 2014 study published in JAMA Internal Medicine, 60 percent of people with a new diagnosis of hypothyroidism and hyperlipidemia had a resolution of their elevated cholesterol levels once their thyroid function was restored. Of those who were treated with levothyroxine, 75 percent did not need lipid-lowering medications within a year of hypothyroidism treatment. This percentage may be higher yet, considering that not everyone in the study had their cholesterol levels rechecked after the restoration of their thyroid function.

From the effects of thyroid replacement on lipids, it can be deduced that the treatment of hypothyroidism most likely lowers heart disease risk, but there is more evidence this is true as well, as levothyroxine has been shown to have positive effects on intimal thickness of the carotid artery (an indirect measure of heart disease risk).

Due to this evidence, professional societies such as the American Association of Clinical Endocrinologists have made recommendations to all physicians who care for people with lipid abnormalities.

Anyone who has a new diagnosis of hyperlipidemia should be screened for hypothyroidism before lipid-lowering medications are recommended. If a diagnosis of hypothyroidism is made, people should be monitored to see if levothyroxine therapy improves cholesterol levels.

In some instances, cholesterol levels will remain elevated despite treatment, and if lifestyle measures such as diet, weight loss, and exercise are not effective, cholesterol-lowering medications may then be considered.

Of note, levothyroxine appears to enhance the effectiveness of statins, ezetimibe, and PCSK9 inhibitors—all options for lowering lipid levels.

Missed Opportunities for Cholesterol Treatment

A lipid panel is a test most doctors use to evaluate cholesterol abnormalities and is recommended for everyone beginning at age 20 (or earlier if there's a family history or other risk factors). This panel tests total cholesterol, LDL ("bad" cholesterol), HDL ("good cholesterol" or essentially the molecule that removes LDL from the body), and triglycerides.

If LDL or triglycerides are elevated, cholesterol medications may be needed, but the level at which this is recommended depends on several factors, including other risk factors for heart disease. For example, one of the statins may be recommended for a person with an LDL of 100 to reduce heart disease if other risk factors are present.

But, given the above, the presence of thyroid disease must also be considered. Unfortunately, many people have undiagnosed hypothyroidism, and far too few doctors are checking people with high cholesterol for thyroid disease.

Treating the Symptom, Not the Cause

Rather than following a treatment plan that incorporates levothyroxine, it appears that many people whose high cholesterol levels might be resolved by treatment of their thyroid disease alone are solely using cholesterol-lowering medications.

Cholesterol Treatments and Thyroid Disease

Just as thyroid medications may affect cholesterol, cholesterol treatments may effect thyroid disease and/or the efficacy of thyroid drugs in several ways:

  • Absorption: Bile acid resins such as Questran (cholestyramine), Colestid (colestipol), and Welchol (colesevelam) may bind to levothyroxine, completely preventing its absorption. These drugs should be taken at least four hours after taking levothyroxine.
  • Muscle pain: This is a common side effect of cholesterol medications known as statins, such as Lipitor (atorvastatin) and Pravachol (pravastatin), as well as PCSK9 inhibitors such as Repatha (evolocumab). People who have hypothyroidism are more likely to experience muscle pain on these drugs than people without thyroid disease. The incidence of muscle pain is higher with Zocor (simvastatin), and lower with Pravachol and Lescol (fluvastatin).
  • Flushing and other hyperthyroidism symptoms: Niacin may be used for people who have a low HDL level. Its side effects, such as flushing, can closely mimic the symptoms of hyperthyroidism. This could be important for people with hyperthyroidism, as well as those who have hypothyroidism, who may wonder if their levothyroxine dose is too high.
  • Thyroid eye disease: On the plus side, a 2018 study found that treating high cholesterol with statin medications reduced the risk of Grave's orbitopathy (eye symptoms such as bulging and more).

Lifestyle Measures That Benefit Both Conditions

There are several lifestyle measures that can be helpful both in lowering cholesterol and reducing the symptoms common with thyroid disease.

  • Diet and weight loss: Weight loss can be challenging with thyroid disease, but even small weight reductions can lower LDL cholesterol levels. In addition, an anti-inflammatory diet may reduce the inflammation associated with autoimmune thyroid disease while lowering cholesterol as well.
  • Exercise: Regular exercise can improve energy levels with hypothyroidism, help maintain bone density with hyperthyroidism, and raise good (HDL) cholesterol levels.
  • Quitting smoking: Smoking is harmful to the thyroid gland and increases the risk of complications of thyroid disease. It also compounds the risk of atherosclerosis (heart disease, strokes, and more) related to high cholesterol.

A Word From Verywell

If you have been newly diagnosed with high cholesterol or other lipid abnormalities and do not have known thyroid disease, be sure to have your doctor screen you for hypothyroidism right away, and especially before taking any cholesterol medications. This can be done with a simple blood test. You may need to be your own advocate and request the test since the JAMA Internal Medicine study noted above found that only around 50 percent of people with newly diagnosed hyperlipidemia were adequately screened for thyroid disease. Likewise, if your cholesterol levels are suddenly improving for no apparent reason, blood tests should be done to check for hyperthyroidism.

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