Can Hypothyroidism Cause Arterial Hypertension?

Systolic blood pressure is excessive in hyperthyroidism

In hyperthyroidism (excessive thyroid function), the systolic blood pressure increases, but the diastolic decreases. Because of the increase in systolic blood pressure, it is often thought that thyroid hormones systematically increase the blood pressure. In hyperthyroidism, thyroid hormones elevate the blood pressure by two actions. First, they make the heart beat quicker. This tachycardia makes the heart pump more blood to the arteries, phenomenon that increases the pressure in the arteries. Second, hyperthyroidism increases the number of adrenergic receptors in the arterial walls. Adrenergic receptors are receptors for adrenaline and noradrenaline, neurotransmitters that increase the blood pressure. Because of this increase in receptors, adrenaline and noradrenaline can better exercise their stimulating action on the blood pressure. 

Both systolic and diastolic blood pressure increase in hypothyroidism

However, studies show that patients with important thyroid deficiency have a significantly higher blood pressure, particularly a higher diastolic blood pressure. When the diastolic blood pressure is higher, the risk of cancer, cardiovascular disease (including ischemic heart disease and atrial fibrillation), stroke, chronic kidney disease, dementia, cancer, etc. increases. Neurologists explain that even slight excessive increases in blood pressure may produce small hemorrhages in the brain, which damage the brain and, by accumulation, may considerably impair the brain function. 

In hypothyroidism, mucopolysaccharides (waste products) accumulate in the walls of the arteries and make these walls rigid. When the blood flows through rigid arteries, it increases. Arteries need to have flexible walls that easily extend each time blood pushed by a heart contraction flows through them. Blood pressure is lower in extensible arteries because they widen their lumen when the blood pressure increases.

Thyroid therapy may considerably reduce blood pressure in hypothyroid patients

A study published in the 1960s showed that thyroid therapy significantly reduced the diastolic blood pressure in more than 90% of hypothyroid patients with arterial hypertension and that the systolic blood pressure completely normalized in 50% of hypothyroid patients. However, the important efficacy of thyroid treatment in reducing the blood pressure in this study might have been due to the use of desiccated thyroid, which was at that time the main thyroid treatment. Desiccated thyroid is richer in the whole gamma of different thyroid hormones: T4, T3, T2, T1, and T0 (the numbers refer to the number of iodine atoms attached to a tyrosine molecule).

More recent studies have confirmed that low thyroid hormone levels within the reference range and below the lower limit of the reference range, and high TSH levels in the serum, tend to be associated with higher systolic blood pressure and even more with higher diastolic blood pressure. Several studies using the less potent thyroxine treatment have shown beneficial results in lowering blood pressure. 

A pinched blood pressure is a typical sign of hypothyroidism

In hypothyroidism, it is typical for the differential blood pressure to be “pinched.” The differential blood pressure is the difference between the systolic and diastolic blood pressures. In general, it is equivalent to half of the systolic blood pressure decreased by approximately 10 mmHg. For people with an average systolic blood pressure of 120 mmHg and diastolic blood pressure of 70 mmHg, the differential blood pressure is around 50 mmHg. In hypothyroidism, the differential is lower than normal, 40 mmHg or less. With a hypothyroid patient, for example, the systolic blood pressure can typically be 130 mmHg and the diastolic blood pressure 90 mmHg, providing a pinched differential of 40 mmHg (130 mmHg - 90 mmHg = 40 mmHg). In healthy individuals, you would expect a greater differential of 55 mmHg (130 mmHg: 2 = 65 mmHg- 10 mmHg = 55 mmHg). In hyperthyroidism (thyroid excess), the opposite is true: The differential blood pressure is usually excessive. The systolic blood pressure can typically be 140 mmHg and the diastolic 40 mmHg, which provides a differential blood pressure of 100 mmHg (140 mmHg - 40 mmHg = 100 mmHg). 

Remember as a general rule that thyroid hormones reduce the blood pressure

Because hyperthyroidism is a rare occurrence, these findings indicate that thyroid hormones in general decrease the blood pressure in people with arterial hypertension by stimulating the elimination of myxedema in the arterial wall, permitting them to be more flexible and widen their lumen when the blood passes through. If thyroid therapy can decrease both the systolic and diastolic blood pressure, it specifically helps to lower the diastolic blood pressure more and may, thus, also reduce the risk of developing diseases associated with high diastolic blood pressure.

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