One in eight women have experienced thyroid dysfunction, such as hypothyroidism or hyperthyroidism.
Thyroid disorders can be responsible for other symptoms such as extreme fatigue, anxiety, blood pressure abnormalities, and other systemic changes.
In this article we will explore more in depth the relationship between thyroid dysfunction and blood pressure changes.
Hypo or Hyper?
The two abnormal disorders for the thyroid are known as: hyperthyroidism and hypothyroidism.
Hyperthyroid is commonly caused by Graves’ Disease and is frequently associated with higher systolic blood pressure or systolic hypertension. (Systolic is the number on top in a blood pressure reading.)
Hypothyroid, or low thyroid, is also called by its own separate autoimmune component, Hashimoto’s Thyroiditis or autoimmune thyroiditis.
Hypothyroid conditions frequently result in high diastolic blood pressure or diastolic hypertension. (Diastolic is the number on the bottom of a blood pressure reading.)
How does Blood Pressure Work & What is Hypertension?
Blood pressure is the measurement of what kind of pressure is being put on your blood vessels (arteries and veins). The top number in a blood pressure reading reveals the pressure on the walls of the vessels when the heart is pumping, systolic blood pressure.
The bottom number represents the pressure on the walls of the vessels when the heart is at “rest” or between beats, diastolic. Blood pressure acts diurnally, as well: higher during awake hours and lower while sleeping. Blood pressure readings consistently above 140/90 are considered hypertensive.
Blood pressure is regulated throughout the body by different systems. The kidneys sense how much blood volume is present and adjust how much fluid they filter in response to current blood volume.
With high blood volume, they will try to reduce blood pressure by filtering more fluid out of the body.
With low blood volume, the kidneys try to keep more fluid “inside.” There is a delicate dance of hormones monitoring and responding to blood volume including renin, angiotensinogen, angiotensin and aldosterone.
Sympathetic Nervous System
Blood pressure is also controlled by the sympathetic nervous system (SNS). This is your fight or flight response and usually includes increased blood pressure, decreased digestion, tachycardia (fast heart rate) and salt retention.
This is the system response driven by the adrenal glands and relying on substances called catecholamines: adrenaline, noradrenaline and cortisol. Cortisol is often associated with stress responses.
When your body activates the SNS, your blood pressure elevates, but it’s only intended to be elevated for a short amount of time. When the danger or stress is gone, everything should return to normal.
Hypothyroid conditions, or undertreated hypothyroidism, results in an inadequate amount of T3 or triiodothyroxine. The thyroid produces T4 and the T4 is changed to T3 throughout the body. T3 is responsible for aiding in many processes throughout the body. Here we will examine its effect on blood pressure through vasodilation or relaxation of the blood vessels.
T3 acts directly on cells to promote vasodilation, especially in the arteries of the extremities. When a hypothyroid state occurs, there is less circulating T3 which is a result of either overt hypothyroidism or subclinical hypothyroidism.
Constriction or stiffening of the vessels occurs with less T3, decreasing the size of the vessels. Blood in less flexible vessels increases the pressure put on the vascular system, elevating the blood pressure, and leads to clinically discernable hypertension.
Physiological Conditions Contributing to Hypertension in Hypothyroid Diseases
Reduced Blood Volume & Cardiac Output
Hypothyroid conditions frequently result in an overall decreased about of blood volume and related decreased cardiac output (how much blood the heart pumps in a given amount of time). This decreased blood volume triggers the kidneys to raise blood pressure.
Reduced blood volume paired with the decrease in available T3 explains the prevalence of diastolic hypertension in hypothyroid patients: lower blood volume results in less blood to pump around, so while the vessels are not as flexible, there is not as much blood being pushed per pump of the heart.
Systolic blood pressure increases, but not as much in relation to diastolic pressure increases. The constant pressure from the blood volume on further restricted vessels is revealed by the diastolic pressure. Over time, without adequate treatment for the hypothyroid condition, both numbers will continue to elevate.
Blood pressure is independent of TSH levels: actual thyroid hormone levels are the indicator. The mechanism for systolic hypertension in hypothyroid disease isn’t quite understood, yet.
Patients with hypothyroidism, whether untreated, undertreated or well controlled, frequently present with obesity. Increases in Body Mass Index (BMI) are themselves an independent risk factor for increased blood pressure and resulting comorbidities (other problems).
Whether or not there is a linear relationship between increases in BMI and increases in blood pressure is not well documented, but above BMI of 21 kg/m2, blood pressure will increase, more notably in women than in men.
The vascular system relies upon multiple pathways to maintain proper dilation. The thyroid hormone T3 is one pathway affecting vasodilation as well as regulation of the parasympathetic nervous system after sympathetic nervous system activation has ceased. Another pathway involves blood sugar levels and the relationship between nitrous oxide release in cells of the vasculature (inside the cells of the blood vessels).
Hyperglycemia, the abnormal condition of having too high a concentration of glucose in the blood (high blood sugar) results in a series of enzyme concentration changes resulting in less nitrous oxide (NO) being released in the blood vessels. A decrease of up to a 67% decrease of NO concentration was noted in animal cells exposed to a hyperglycemic state. NO contributes to relaxation in blood vessels so a decrease in NO concentration causes the blood cells to constrict and elevating blood pressure.
Hypothyroid patients commonly present with hyperglycemia, reduced levels of NO and the resulting hypertension. Hyperglycemic states can be well controlled with adequate and therapeutic levels of thyroid hormone replacement, diet and exercise.
Sympathetic Nervous System Activation
During the hypothyroid state, the sympathetic nervous system (SNS) will activate in order to attempt to maintain homeostasis. The SNS is meant as a short term solution to a short term stressor. Activation, both short term and long term have effects on blood pressure.
When the SNS is in good condition it will elevate the blood pressure in an attempt to keep blood flowing to the extremities. When the stressor is extinguished, the parasympathetic nervous system brings the body back to homeostasis.
Meanwhile, the hypothyroid state causes the SNS to stay activated. The blood vessels in the extremities constrict due to low T3, further contributing to the hypertension present but the fact this is SNS driven can be missed because the usual symptom of tachycardia (fast heart beat) is absent in the hypothyroid patient.
If the thyroid is not treated or is undertreated, the SNS can keep trying to make up the difference, resulting in adrenal insufficiency. This is a low blood pressure state often accompanied with fainting and almost always decreased renal function. Without treatment it is fatal.
Effective Treatment Solutions
Hypertension in hypothyroid patients is resolved upon proper dosing with replacement thyroid hormone, T4. In all cases, once therapeutic levels of replacement are reached the hypertensive condition is no longer detected. Lifestyle and diet improvements are crucial to continued good health due to the propensity for hypothyroid patients to present with obesity and hyperglycemia.
The regulation of blood pressure in normotensive individuals is a complex dance of physiological conditions and hormones which becomes increasingly complex with the addition of the hypothyroid condition. A combination of proper screening for subclinical hypothyroidism, therapeutic hormone dosing and appropriate diet and lifestyle changes are effective treatment protocols to alleviate the hypertensive condition.
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Accessed via web on 11 APR 2019. https://eprints.ugd.edu.mk/185/1/DYSLIPIDAEMIA%20AND%20HYPERTENSION%20IN%20PATIENTS%20WITH.pdf
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