Hypothyroidism → Cardiovascular

• Hypothyroidism, overt: ↓ thyroid hormones (T4, T3)

• Hypothyroidism, subclinical: ↑ TSH, normal FT4 (i.e. while measured thyroid hormone is “normal,” high TSH serves as a marker of ↓ intracardiac T3 availability & thyroid hormone receptor activity)

Acute (Rare)

• (Pre)syncope (bradyarrhythmias ⊕ ↓ β1)
• Torsades (acquired long QT syndrome ⊕ PVC)
• Decompensated CHF (↓ contractility vs. pericardial effusion)
• Vasospasm → PAD mimicker

Chronic (Common)

• ↓ Exercise capacity
• Asymptomatic sinus bradycardia
• Orthostatic hypotension
• Diastolic hypertension
• Atherosclerosis → PAD

Normal Physiology ✅

• Within the heart, T3 enters cardiac myocytes & activates thyroid hormone nuclear receptors, ultimately upregulating β1 receptors (↑ contractility, ↑ HR) & enzymes necesssary for calcium flux (↑ compliance → ↑ diastolic filling)

• Peripherally, T3 stimulates endothelial NO synthase & therefore results in vascular smooth muscle vasodilation with resultant ↓ systemic vascular resistance

Pathophysiology ❌

• ↓ T3 (the bioactive thyroid hormone) results in the inverse effects of normal T3 function, as well as pericardial capillary leak (↓ lymphatic drainage)

• Notably, cardiovascular symptoms & signs in patients with hypothyroidism are uncommon & when present, often subtle

• ↑ TSH itself may also serve a pathophysiologic function via effects on cardiac myocyte resting membrane potentials & ↑ parasympathetic tone

• Cardiovascular effects of hypothyroidism

• Thyroid Disease and the Heart

• Reversible Bradycardia and Transient Loss of Consciousness With Undiagnosed Hypothyroidism: A Case Report

• Bradyarrhythmia as a presenting feature of subclinical hyperthyroidism

• Thyroid stimulating hormone directly modulates cardiac electrical activity

• Differences in Heart Rate Profile During Exercise Among Subjects with Subclinical Thyroid Disease

• Heart rate variability and QT dispersion in patients with subclinical hypothyroidism

• The Vasospastic Thyroid: Bilateral Leg Pain as a Vascular Manifestation of Hypothyroidism