Again, Thyroid Hormones and The Heart
Yesterday, we sat with a friend watching our daughters play volleyball and in conversation she said, "They keep telling me my thyroid is fine but I know something isn't right, my hair is thinning, I'm freezing all the time, I have no energy, I have felt terrible for so long, but my doctors keep telling me my labs are normal." Thyroid hormone is one of our absolute favorite hormones because within our bodies, it is nearly involved in every system; this paper touches on thyroid's impact on the heart. There are so many papers outlining this importance.
Thyroid Hormones: Synthesis and Biological Activity
Thyroid hormones (THs) like thyroxine (T4) and triiodothyronine (T3) are instrumental players in our body's orchestra. It's fascinating how their synthesis in the thyroid gland is steered by the thyroid-stimulating hormone (TSH). While around 85% of T4 takes its origin from the thyroid gland, it later finds itself converted to T3 in various locales like the liver, kidneys, and skeletal muscles, with the enzyme 5′-monodeiodinase playing the maestro. And then there’s reverse T3 (rT3), an inactive derivative of T4. Notably, a vast majority of these THs get bound to transport proteins, rendering only a minuscule portion biologically active.
Cardiac Cellular Dynamics and Thyroid Hormones
Peeking into the cellular landscape, it's evident that THs play their roles with precision. They are intimately intertwined with cardiac cells. The thyroid hormone nuclear receptors (TRs) often bind to thyroid hormone response elements (TREs), primarily as heterodimers, influencing gene transcription. But here's the catch: only T3 gets an entry pass into the myocyte. Absence of T3 means certain genes, like β-myosin heavy chain and phospholamban, get triggered. Meanwhile, when T3 is around, it flips the switch on other key cardiac genes, ultimately affecting heart contraction and relaxation. Interestingly, T3 also extends its touch to the vascular realm, influencing the renewal of endothelial integrity and modulating vascular response mechanisms.
Immediate Cardiovascular Impacts of T3
Moreover, thyroid hormones, particularly T3, have a rapid impact on the heart and vascular system. These effects can be felt almost instantly, altering ion channels and influencing heart rate, left ventricular contractility, and venous tone. THs even influence the systemic vascular resistance, leading to enhanced cardiac output. To put things in perspective, while some of these T3 effects take just seconds, others mediated by nuclear pathways can stretch up to two hours!
Here's a snapshot of THs' effects on hemodynamics:
Systematic Vascular Resistance (SVR): Decrease
Diastolic Blood Pressure (DBP): Decrease
Afterload: Decrease
Cardiac Inotropy-Chronotropy: Increase
Basal Cardiac Output (CO): Increase
Blood Volume Preload: Increase
Dysfunction Dances: Hyperthyroidism vs. Hypothyroidism
Diving deeper, the relationship between thyroid function and heart health becomes even more evident. Thyroid dysfunction has remarkable consequences. Hypothyroidism brings along a suite of issues including hypertension, atherosclerotic plaque progression, and more. Hyperthyroidism, or the abnormal production of thyroid hormone from an autoimmune process called Grave's disease, on the flip side, ushers in problems like increased blood pressure, pulmonary hypertension, and issues with the atrioventricular valve. Some hyperthyroid patients may even experience chest pain, while others can showcase symptoms of heart failure.
And there's more! Our heart's rhythm and rate are sensitive to thyroid fluctuations. Minor thyroid status changes can precipitate significant heart irregularities. Arrhythmias, atrial flutter, and fibrillation are frequent in hyperthyroid patients. It seems that these responses may be related to the other "unknown" effects from Grave's disease, because these issues are typically not seen when high levels of thyroid hormone are supplemented.
Contrastingly, hypothyroidism often brings along slowed heart rhythms. Multiple studies highlight the mixed antiarrhythmic and proarrhythmic effects of THs, emphasizing their influence on heart cell electrogenesis.
Lamprou, V, Varvarousis, D, Polytarchou, K, Varvarousi, G and Xanthos, T. The role of thyroid hormones in acute coronary syndromes: Prognostic value of alterations in thyroid hormones. Clin Cardiol. 2017; 40: 528–533. https://doi.org/10.1002/clc.22689
We didn't always see things the way we do today. . . but we do now. People are not their lab test values! Jason & Rita...aka Dr. De Leon and Dr. Gillespie.
