The effect of thyroid hormones on blood pressure
Thyroid hormone has well-recognized effects on the cardiovascular system and blood pressure regulation. Blood pressure is altered across the entire spectrum of thyroid disease. The effects of hyperthyroidism include increased cardiac output, contractility, tachycardia, widened pulse pressure, decreased systemic vascular resistance, and increased basal metabolic rate. The manifestations of hypothyroidism are in marked contrast to those of hyperthyroidism and include decreased cardiac output, narrow pulse pressure, increased systemic vascular resistance, and decreased metabolic rate. Although thyroid hormone affects almost all tissues of the body and mediates changes in homeostasis, adaptations of the cardiovascular system can result in changes in blood pressure to accommodate the new demands on the system. In this paper, we review the direct and indirect thyroid hormone-mediated effects on blood pressure.
Systolic or Diastolic Hypertension?
Blood pressure is the force of blood pushing against the walls of arteries as the heart pumps blood. High blood pressure, also referred to as hypertension, is when that force is too high and begins harming the body. If left untreated, it willl eventually cause damage to the heart and blood vessels.
Your blood pressure is measured in two numbers: The top systolic blood pressure measures the force pushing against artery walls when the heart is contracting. The bottom diastolic blood pressure measures pressure in the arteries when the heart is resting between beats.
Normal blood pressure levels are 120 mmHg/80 mmHg or lower. At risk levels are 120-139 mmHg/80-89 mmHg. Readings of 140 mmHg/90 mmHg or higher are defined as high blood pressure.
Secondary blood pressure
Secondary hypertension (secondary high blood pressure) is high blood pressure that’s caused by another medical condition. Secondary hypertension can be caused by conditions that affect your kidneys, arteries, heart or endocrine system. Secondary hypertension can also occur during pregnancy.
Secondary hypertension differs from the usual type of high blood pressure (primary hypertension or essential hypertension), which is often referred to simply as high blood pressure. Primary hypertension has no clear cause and is thought to be linked to genetics, poor diet, lack of exercise and obesity.
Proper treatment of secondary hypertension can often control both the underlying condition and the high blood pressure, which reduces the risk of serious complications — including heart disease, kidney failure and strokes.
Association between hypothyroidism and hypertension
For most, high blood pressure, or hypertension is can be attributed to family history or lifestyle. For some, however, high blood pressure is the result of either an overactive or underactive thyroid.
Conversely, hypothyroidism occurs when the thyroid gland is underactive and does not produce enough hormones. Hypothyroidism can weaken the heart muscle and heart rate, reducing the heart’s pumping capacity and increase the stiffening of blood-vessel walls. The combination of these changes can lead to hypertension.
If a thyroid disorder is the reason for high blood pressure, adjustments in diet and lifestyle won’t resolve the problem and medications may not be effective. If a patient’s blood pressure does not respond to conventional treatment, a doctor can perform tests to see if the thyroid is the cause.However, hypertension is not a typical sign of hypothyroidism.
Hypothyroidism has been recognized as a cause of secondary hypertension. Previous studies on the prevalence of hypertension in subjects with hypothyroidism have demonstrated elevated blood pressure values. Increased peripheral vascular resistance and low cardiac output has been suggested to be the possible link between hypothyroidism and diastolic hypertension.
Basic causes of primary hypothyroidism are autoimmune, silent, postablative, goitrous, athyreotic and nonautoimmune (e.g., Riedel’s), and subacute thyroiditis.[3] Chronic autoimmune lymphocytic thyroiditis (Hashimoto’s disease) is the most common cause of thyroid gland dysfunction.
Hypertension, most commonly diastolic, is increased in patients with hypothyroidism because of increased peripheral vascular resistance. Hypertension is caused by hypothyroidism in 3 percent of patients with high blood pressure. Hypercholesterolemia and an increase in fatty acids are also associated with low thyroid function, thus increasing the risk of cardiovascular disease.
Increase in Peripheral Vascular Resistance
found that diastolic BP correlated significantly with thyroxine (T4) and 3,5,3′-triiodothyronine (T3) in slightly hypothyroid females over 50 years of age.[2]
triiodothyronine represents the metabolically active thyroid agent that possibly has a vasodilatory effect on the vascular muscle cells.[16] Hypothyroidism and T3 deficiency are associated with peripheral vasoconstriction.Hypothyroidism has been associated with increased arterial stiffness.
On the other hand, the increased systolic and diastolic BP could induce changes in the arterial wall, reducing elasticity and increasing stiffness.
Adequate thyroid hormone replacement therapy successfully reduced BP, supporting the secondary cause of hypertension in patients with hypothyroidism.
The authors conclude that hypothyroidism causes aortic stiffness and hypertension (usually diastolic). Thyroid hormone therapy decreases aortic stiffness, promoting decreased blood pressure in about 50 percent of these patients. Antihypertensive treatment further improves aortic elasticity and can decrease blood pressure among patients with hypertension and hypothyroidism whose blood pressure does not drop as thyroid function is normalized. Replacement of lacking thyroid hormones reduces high blood pressure (BP) and total cardiovascular risk.
References
https://www.medscape.com/viewarticle/733788_5
https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(17)33320-2/fulltext
https://pubmed.ncbi.nlm.nih.gov/14594573/#:~:text=Blood%20pressure%20is%20altered%20across,and%20increased%20basal%20metabolic%20rate.
https://www.rush.edu/news/6-high-blood-pressure-facts
https://www.mayoclinic.org/diseases-conditions/secondary-hypertension/symptoms-causes/syc-20350679
https://www.aafp.org/afp/2002/0901/p851a.html#:~:text=Hypertension%2C%20most%20commonly%20diastolic%2C%20is,patients%20with%20high%20blood%20pressure.
