Introduction:
Hypothyroidism is caused when the levels of thyroid hormones decrease. Thyroid hormones play a role in many important bodily functions, including the normal functioning of the heart. Hypothyroidism may affect the function of the heart. It is seen in 4 %—10 % of the population.
What Is Hypothyroidism?
The thyroid gland is an important organ that is responsible for metabolism. Metabolism is a process by which food is converted into energy, which all the body's cells use to function normally. The thyroid gland secretes thyroxine (T4), and triiodothyronine (T3).
T4 and T3 control the body's metabolism. They work throughout the body and help cells utilize energy. They also control other functions, like body temperature and heart rate.
Hypothyroidism can affect any age, gender, or ethnicity. Women are more commonly affected, especially after 60 years. Women are affected after menopause.
What Are the Impacts of Hypothyroidism on Heart Health?
The thyroid gland is responsible for the smooth functioning of most body organs, including the heart. It can also influence heartbeat, blood pressure, and cholesterol levels.
Malfunctioning of the thyroid can cause heart-related diseases or may worsen an existing heart disease. It is estimated that 6 % of people in the United States (U.S.) suffer from thyroid problems. 80 % of these are affected by hypothyroidism.
When thyroid levels drop, the body's processes slow. Symptoms include weakness, weight gain, cold intolerance, constipation, and dry skin. Hypothyroidism can also affect the heart and circulatory systems.
Hypothyroidism is related to decreased cardiac output. The reason for decreased cardiac output may be impaired relaxation of the vascular smooth muscle and a decrease in the availability of endothelial nitric oxide. This causes increased arterial stiffness, leading to increased systemic vascular resistance.
These changes occur due to reduced sarcoplasmic reticulum Ca2+-ATPase and increased phospholamban expression. Phospholamban can inhibit ATPase.
Thyroid hormones can influence the renin—angiotensin- aldosterone system. Renin substrates are formed in the liver under the influence of T3. Decreased T3 in hypothyroidism leads to increased diastolic blood pressure, narrowed pulse pressure, and decreased renin levels. The increased diastolic blood pressure is considered sodium-sensitive.
T3 can help increase erythropoietin (a hormone that can produce red blood cells). In hypothyroidism, normocytic normochromic anemia can be observed. Thyroid hormones play a role in pacemaker-related genes in cardiomyocytes. These mechanisms can increase the heart rate in the presence of T3 and decrease the heart rate with lowered levels of thyroid hormones.
Various changes in lipid parameters can be observed in hypothyroidism. These include increased total cholesterol levels, low-density lipoproteins (LDL) cholesterol, and apolipoprotein B.
There is a decreased expression of hepatic LDL receptors and cholesterol-⍺-monooxygenase activity in hypothyroidism. These hepatic LDL receptors and cholesterol-⍺-monooxygenase are responsible for the breakdown of cholesterol. Due to their reduced activity, decreased LDL clearance can be observed. Increased levels of C-reactive protein and homocysteine can be observed.
Thyroid hormones are responsible for endothelial function, which is mediated by thyroid hormone receptors (THR)—1 and THR-β. The activation of the (THR)—1 receptor increases blood flow to the heart, decreases coronary resistance, and increases nitric oxide production in endothelial and vascular smooth muscle cells.
Activation of THR-β can induce angiogenesis by initiating the mitogen-activated protein kinase pathway. In severe hypothyroidism, pericardial effusion can occur. This condition is caused by increased capillary permeability and reduced lymphatic drainage from the pericardial space.
Heart Failure and Hypothyroidism:
Hypothyroidism can affect the heart's contractility and damage the relaxation of heart muscles. Diastolic hypertension (increased blood pressure) with coronary artery disease may impair myocardial diastolic function. These actions decrease cardiac output, heart rate, and stroke volume.
In advanced heart failure and after myocardial infarction, the conversion of T4 to T3 decreases. The decrease in T3 levels affects myocardial contractility and muscle remodeling. It was thought that mortality may result from decreased T3 levels among individuals with heart diseases.
Arrhythmia and Hypothyroidism:
Atrial fibrillation may be caused by hyperthyroidism and hypothyroidism. Hypothyroidism may be beneficial for ventricular fibrillation. Hence, it is thought that hypothyroidism is associated with a decreased probability of cardiac arrhythmias.
Subclinical Hypothyroidism and the Heart:
Subclinical hypothyroidism is characterized by slightly higher levels of (thyroid-stimulating hormone (TSH) than normal TSH levels. The severity of subclinical hypothyroidism depends on the rise in TSH levels. A rise in TSH levels of 4 mIU/L to 10 mIU/L is considered mildly elevated. If the levels of TSH rise to more than 10 mIU/L, it is considered severe subclinical hypothyroidism.
Subclinical hypothyroidism can progress to hypothyroidism eventually. The annual risk of progression from subclinical hypothyroidism to hypothyroidism is 1 % to 5 %. This depends on TSH levels and thyroid antibodies.
It was observed that all changes, such as changes in cardiovascular arterial compliance, increased diastolic blood pressure, endothelial changes, and increased lipid levels, can occur in subclinical hypothyroidism.
Researchers have found that there is a link between subclinical hypothyroidism and ischemic heart disease. A study observed that mortality due to all causes, and cardiovascular death, is considered to be an increased risk with subclinical hypothyroidism.
A study indicated that subclinical hypothyroidism is related to an increased risk of coronary heart disease and mortality. This was noticed among those individuals with TSH levels of more than 10 mIU/L.
Age and TSH:
Heart diseases increase with age, and studies have observed that TSH levels increase with advancing age. Older individuals were also found to have increased levels of TSH in the absence of thyroid disease.
Conclusion:
The thyroid gland is an important gland that maintains many body functions. Thyroid hormones can impact heart health. Various studies have found that hypothyroidism has many effects on the heart. Hence, it becomes important to know about the thyroid and its diseases, especially hypothyroidism, and its impact on the heart if it is not treated. Knowing helps in identifying the disease early. Early diagnosis always leads to effective treatment. This, in turn, helps achieve a good quality of life.
