Don't forget to check thyroid function for dyslipidemia

Ms. Zhang, 58 years old, was diagnosed with hyperlipidemia (TC: 7.05 mmol/L, TG: 4.32 mmol/L, HDL-C: 1.90 mmol/L, LDL-C: 3.45 mmol/L) at a local hospital six months ago. Ms. Zhang thought it was an adverse reaction caused by lipid-regulating drugs, so she stopped taking lipid-regulating drugs for a while, but the symptoms of fatigue and weakness did not improve, so she came to the hospital again.

During the consultation, the doctor found that Ms. Zhang's expression was indifferent and tired, she did not talk much, her heart rate slowed down on auscultation, and when asked about her appetite and stool, she replied that she was often constipated and her appetite was poor, so she was advised to have her thyroid function checked.

The test results: T3, 0.52 nmol/L; T4, 31.3ug/dl; FT3, 1.63 pmol/L; FT4, 0.37 pmol/L; TG-Ab, 15.7 IU/ml; TPO-Ab, 22.6 IU/ml; TSH, 55.37 mIU/L, diagnosed as hypothyroidism.

The doctor said that Ms. Zhang's abnormally high blood lipids and uncomfortable symptoms were caused by hypothyroidism (hypothyroidism), and it was not possible to rely on lipid-regulating drugs alone, so she was given levothyroxine tablets, 50µg, QD, orally, and after 1 month of treatment, her condition improved and her blood lipids gradually returned to normal.

In general, people have the impression that dyslipidemia is mostly related to an unhealthy diet leading to and lack of exercise leading to overweight or obesity, in addition to heredity, smoking, alcoholism, and postmenopausal women are also important causes of dyslipidemia. But, in addition to these factors, dyslipidemia can also be secondary to other diseases, among which hypothyroidism is one of the causes.

Why does hypothyroidism increase blood lipids?

We know that the main function of the thyroid gland is to secrete thyroid hormones, and thyroid hormones have a great influence on lipid metabolism. They can, on the one hand, promote the synthesis of cholesterol by the liver and release it into the blood, increasing the blood cholesterol content; on the other hand, they also promote the excretion of cholesterol and its metabolites, reducing the blood cholesterol content. Under normal conditions, the body regulates blood lipid levels in a relatively balanced state by the number of thyroid hormones secreted.

When hypothyroidism occurs, although both cholesterol synthesis and cholesterol excretion are reduced, thyroxine is less effective in degrading cholesterol than in synthesizing it, resulting in a relative increase in overall blood lipid concentrations, and primarily in low-density lipoprotein cholesterol (LDL-C) levels.

(1) Total cholesterol concentrations are often elevated: when plasma thyroid hormone levels are inadequate, cholesterol synthesis in the liver increases, and total plasma cholesterol concentrations are often elevated, above 6.5 mmol/L.

(2) The increase in LDL cholesterol concentration is mainly related to two factors: (1) the increase in the rate of LDL cholesterol synthesis and the decrease in the activity of HMG CoA reductase, the rate-limiting enzyme of cholesterol synthesis; in hypothyroidism, the rate of LDL production in the body increases, which can also cause an increase in plasma LDL-C levels. (2) The rate of LDL cholesterol catabolism is reduced, and the activity of LDL receptors on cell membranes is decreased. In hypothyroidism, LDL receptor activity on hepatocyte membranes is reduced, which can cause impairment of LDL receptor-dependent degradation pathways in vivo, thus causing an increase in plasma LDL-C and apolipoprotein B (apo B) levels.

(3) Decrease in HDL cholesterol concentration: LDL cholesterol and HDL cholesterol are mostly synthesized in the liver and there are competing mechanisms. As the synthesis of LDL increases, the production of HDL naturally decreases.

(4) Elevated triglyceride concentration: In hypothyroidism, the direction of change in plasma triglyceride concentration is uncertain and can increase, normalize or decrease, depending on the degree of hypothyroidism. However, triglyceride levels are generally elevated in patients with more severe diseases.

Dangers of elevated blood lipids in hypothyroidism

The heart is an important target organ for thyroid hormones, and thyroid hormones affect the heart rate and myocardial contractility through direct or indirect effects, so thyroid disease and cardiovascular disease are closely related.

Hypothyroidism is one of the causes of pericardial effusion and cardiomyopathy. High blood lipids caused by hypothyroidism are also one of the risk factors for cardiovascular disease, and high blood lipids, especially LDL and Apo B, promote atherosclerosis, cause ischemic heart disease, and increase the incidence of cardiovascular events.

What should be done for hypothyroid patients with dyslipidemia?

For patients with elevated lipids, after receiving a thyroid function test, if it is determined to be caused by hypothyroidism, taking lipid-lowering drugs alone is not very meaningful in terms of therapeutic effect, because generally speaking, the disorder of plasma lipoprotein metabolism in hypothyroidism is reversible, so there is no need to rush to take lipid-lowering drugs for treatment.

(1) After patients take thyroid hormone for replacement therapy, thyroid hormone can increase the activity of plasma lipoprotein lipase and liver triglyceride enzyme, which can lower triglyceride to normal and bring back the level of high-density lipoprotein cholesterol, which can promote the transformation of cholesterol to bile acid and promote the gradual return of low-density lipoprotein cholesterol and total cholesterol level to normal.

At present, the first-line drug for the clinical treatment of hypothyroidism is levothyroxine sodium tablets. The dosage of levothyroxine sodium tablets is determined based on the results of the thyroid function test, combined with the patient's age and the severity of the disease. After taking the drug to replenish the thyroid hormone to normal levels, the blood lipids will be rechecked after 3-6 months. Generally speaking, as hypothyroidism is controlled, the blood lipid level can gradually return to normal.

It is important to remember that treatment of hypothyroidism often requires lifelong medication. You should not stop taking the medication or reduce the dosage on your own because the clinical symptoms have reduced or disappeared and the blood lipid level has normalized. Therefore, patients with hypothyroidism should go to the hospital for check-ups on time during the medication period, and should not stop taking the medication because their blood lipids have returned to normal, to avoid the recurrence of the disease.

(2) There are many reasons for elevated blood lipids, and hypothyroidism is only one of them. If the lifestyle habits are not good, even if the hypothyroidism is well controlled, the blood lipids will not return to the normal range, so lifestyle adjustment is also very important. In addition to moderate exercise, keeping in shape, and avoiding obesity, dietary adjustment is a very important aspect of the control of lipid abnormalities caused by hypothyroidism. Patients with hypothyroidism can eat more iodine-rich foods to increase the synthesis of thyroxine, such as seaweed, seaweed, and other seafood, and timely supplementation of high-quality protein, such as eggs, dairy, fish, etc., and also intake of rich vitamins, eat a variety of fresh Avoid foods that may lead to goiter, such as cabbage, cabbage, rape, parsley, cassava, walnuts, potatoes, etc., to avoid goiter; try to eat less cholesterol-containing foods, such as animal offal, to avoid the occurrence of vascular sclerosis and kidney disease; avoid salty foods, such as preserved fish, pickles, etc.

(3) Patients with hypothyroidism mostly have dyslipidemia, mainly manifested as increased total cholesterol and LDL-C concentration, and a few patients have hypertriglyceridemia. In the treatment, based on the above diet modification and exercise, if the patient still has dyslipidemia after giving thyroid hormone treatment for 3-4 months to control TSH in the target range, then it may not be simple hypothyroidism-induced dyslipidemia, and specific lipid-lowering treatment is needed, and lipid-lowering drugs can be considered. However, it should be noted that hypothyroidism can lead to increased susceptibility to statin-associated myopathy and induce hypothyroid myopathy, so the patient needs to be closely monitored for muscle pain, weakness, and other manifestations when using statin lipid-lowering drugs, and creatine kinase should be checked if necessary.

Despite the strong association between hyperlipidemia and hypothyroidism, there are still many patients diagnosed with hyperlipidemia who are not screened for hypothyroidism in clinical practice, which indicates that the current awareness of hyperlipidemia is still focused on primary hyperlipidemia due to overnutrition, lack of exercise, obesity, and age and genetic factors, while the awareness of secondary hyperlipidemia caused by hypothyroidism is insufficient This situation may result in the underdiagnosis or misdiagnosis of hyperlipidemia caused by hypothyroidism.

Given this, it is recommended that patients who are clinically found to be hyperlipidemic should undergo thyroid function tests promptly if they are not well treated with lipid-lowering drugs or if they have some symptoms related to hypothyroidism, such as fatness, swelling, weakness, shortness of breath, constipation, hair loss, depression, etc.