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Thyroid hormone and the heart
1Department of Medical Physiology, Internal Medicine, Faculty of Health Sciences, University of Stellenbosch, South Africa.
Insights
Thyroid hormone significantly impacts heart function, and its imbalances can lead to heart disease. Amiodarone, a heart rhythm drug, can disrupt thyroid hormone levels, causing thyrotoxicosis.
Area of Science:
- Endocrinology
- Cardiology
- Pharmacology
Background:
- Thyroid hormone plays a crucial role in cardiovascular function, influencing myocardial gene expression related to calcium handling, impacting systolic and diastolic function.
- Abnormalities in thyroid hormone production, both excess and deficiency, contribute to cardiovascular morbidity and clinical cardiac diseases.
- Thyroid hormone affects peripheral vascular smooth muscle tone and the interaction between the left ventricle and arterial system.
Purpose of the Study:
- To elucidate the cardiovascular effects of thyroid hormone and the impact of its production abnormalities on cardiac health.
- To understand the mechanisms by which amiodarone therapy can induce thyroid dysfunction, specifically amiodarone-induced thyrotoxicosis (AIT).
- To highlight the importance of monitoring thyroid function in patients undergoing amiodarone treatment.
Main Methods:
- Analysis of molecular changes induced by thyroid hormone to understand its role in cardiac pathogenesis.
- Review of amiodarone's effects on thyroid hormone production and thyroid function tests.
- Classification of amiodarone-induced thyrotoxicosis (AIT) into Type I and Type II.
Main Results:
- Excessive thyroid hormone leads to increased cardiac output and work efficiency but reduced cardiac reserve.
- Amiodarone can cause both hyperthyroidism and hypothyroidism, with AIT classified into two types based on pathogenesis.
- Amiodarone alters thyroid function tests in euthyroid patients by inhibiting T4 to T3 conversion, leading to decreased T3 and increased T4 levels.
Conclusions:
- Thyroid hormone is integral to cardiovascular homeostasis, and its dysregulation contributes to heart disease.
- Amiodarone therapy requires careful thyroid function monitoring due to its potential to induce thyrotoxicosis and alter thyroid test results.
- Baseline and regular thyroid function tests are essential before and during amiodarone treatment to guide patient management.
Abstract:
Thyroid hormone has important cardiovascular effects, and abnormalities of its production cause cardiovascular morbidity. The role of both excessive and insufficient thyroid hormone production in the pathogenesis of clinical cardiac diseases can be deduced from thyroid hormone-induced molecular changes. Thyroid hormone regulates the expression of myocardial genes regulating the handling of calcium, which affects both systolic and diastolic myocardial function. Thyroid hormone also has indirect and direct effects on peripheral vascular smooth muscle tone, and alters the coupling of the left ventricle and arterial system. Excessive production of thyroid hormone results in an increased cardiac output as well as increased cardiac work efficiency, but reduced cardiac reserve. Amiodarone therapy for cardiac rhythm can cause both hyper- and hypothyroidism. Amiodarone-induced thyrotoxicosis (AIT) can be due to either excessive thyroid hormone production (type I AIT) or thyroid hormone release due to an inflammatory condition (type II AIT). Classification of AIT is helpful in guiding therapy. Amiodarone causes changes in the thyroid function tests of euthyroid patients on therapy--it inhibits the conversion of T(4) and T(3), which results in decreased T(3) and slightly increased T(4) serum levels in euthyroid patients. Baseline thyroid functions should therefore be determined before starting amiodarone therapy, and at 6-monthly intervals thereafter.