Can hypothyroidism (underactive thyroid) cause hypertension (high blood pressure)?

Hypothyroidism and Hypertension

Yes, hypothyroidism commonly causes hypertension, particularly diastolic hypertension, and this is often reversible with thyroid hormone replacement therapy.

Mechanism and Prevalence

Hypothyroidism causes hypertension through several well-established mechanisms:

• Increased systemic vascular resistance can rise by as much as 50%, leading to elevated blood pressure, particularly diastolic pressure 1
• Diastolic hypertension is the hallmark of hypothyroid-induced hypertension due to increased peripheral resistance 1
• Prevalence is significant: hypothyroidism was found in 3.6% of consecutive hypertensive patients referred for evaluation, making it a relatively common secondary cause 2

The 2017 ACC/AHA Hypertension Guidelines explicitly recommend measuring thyroid-stimulating hormone (TSH) in all newly diagnosed hypertensive patients as part of basic laboratory testing to detect hypothyroidism and hyperthyroidism, both of which are remediable causes of hypertension 3

Reversibility with Treatment

Treatment with levothyroxine normalizes blood pressure in approximately 30-50% of patients with hypothyroid-induced hypertension:

• In one study, 40% of patients developed diastolic hypertension (>90 mmHg) after radioiodine-induced hypothyroidism, and restoration of euthyroidism normalized blood pressure in 56% of these hypertensive patients 2
• Among 688 hypertensive patients screened, 1.2% had hypothyroidism as the primary cause of their hypertension, with complete normalization of blood pressure after thyroid replacement 2
• Complete blood pressure normalization occurred in 50% of patients treated with levothyroxine, while the remaining 50% showed only partial improvement 4

Structural Vascular Changes

Aortic stiffness is a critical factor determining whether hypertension will fully resolve:

• Patients with hypothyroidism and hypertension have significantly increased aortic stiffness compared to controls 4
• Those with persistent hypertension despite thyroid replacement had higher baseline aortic stiffness (24.0 ± 4.1) compared to those whose blood pressure normalized (13.7 ± 3.2) 4
• Aortic stiffness index was independently associated with incomplete normalization of blood pressure (odds ratio = 1.99) 4

Clinical Approach

Screen all hypertensive patients for hypothyroidism with TSH measurement 3:

• This is part of the basic laboratory workup recommended by the ACC/AHA for all newly diagnosed hypertensive patients 3
• Particularly important in patients with diastolic hypertension and other hypothyroid symptoms 1

Treatment strategy:

• Initiate levothyroxine replacement immediately for documented hypothyroidism with hypertension 1
• Start at low doses in patients with cardiac disease to avoid precipitating angina or arrhythmias 1, 5
• Monitor blood pressure response over 2-3 months as thyroid hormone levels normalize 4
• If hypertension persists after achieving euthyroidism, this likely reflects irreversible vascular changes (increased aortic stiffness) and requires standard antihypertensive therapy 4

Important Caveats

Avoid overtreatment with levothyroxine, as this can cause:

• Marked hypertension and tachycardia 5
• Atrial fibrillation, particularly in elderly patients 1, 5
• Angina and other cardiovascular complications 1, 5

The hypertension in hypothyroidism is typically characterized by:

• Elevated diastolic pressure more than systolic 1, 2
• Low plasma renin activity, consistent with sodium retention 6
• Bradycardia rather than tachycardia 1

Younger patients are more likely to experience complete blood pressure normalization with thyroid replacement compared to older patients with more established vascular disease 4, 2