The Most Ignored Cause of Heart Disease: The Thyroid-Heart Connection

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• 🫀 Thyroid Hormones and Your Heart

• 🦋 Testing Thyroid Levels

• ❤️ Strategies for Cardiovascular Health

The heart is one of the body’s most protected organs—biologically, metabolically, hormonally.

• Unlike other tissues, the heart is uniquely designed to resist disease, generate immense amounts of energy, and sustain life. It beats around 100,000 times a day, pumping over 2,000 gallons of blood daily, and remarkably, it almost never develops cancer, infections, fibrosis, fat accumulation, or fatigue.

• The heart is an energetic organ, first and foremost. It’s not just a pump; it’s a muscle that runs on a continuous flow of energy and oxygen. Unlike skeletal muscles, which can rest and recover, the heart must sustain its contractions, for an entire lifetime. This demand requires efficient energy production, a steady oxygen supply, and finely tuned metabolic regulation to ensure it never skips a beat.

• Your heart cells are with you for life. Unlike your skin, liver, or gut, which constantly regenerate, the vast majority of your cardiac muscle cells (cardiomyocytes) are the same ones you were born with. The turnover rate for heart cells is only about 1% a year, and this rate decreases with age.

• Even though heart cells rarely regenerate, they constantly renew their mitochondria, the structures responsible for producing energy. Each heart cell contains thousands of mitochondria, making up nearly 40% of the cell’s volume—more than any other tissue in the body. These mitochondria work continuously to generate energy to ensure the heart never stops beating.

Despite all the body’s attempts to protect it, cardiovascular problems are extremely common. Heart disease is the leading cause of death in the United States. In 2016, approximately 121.5 million adults in the United States—nearly 48% of the adult population—were living with some form of cardiovascular disease and rates are increasing.

And it isn’t the result of simple mechanical wear and tear. The heart is designed to last a lifetime, with built-in mechanisms to sustain energy production, resist fatigue, and maintain circulation.

Many people are told to take a statin, limit their intake of red meat and fat, and exercise and they are set for life. But this simply isn’t the case. Heart disease, metabolic dysfunction, and chronic inflammation are far more complex than just managing cholesterol levels.

Even thought the focus has been on lowering LDL (“bad”) cholesterol, but this oversimplifies the real root cause of cardiovascular disease. Metabolic inefficiency, hormone imbalance, chronic inflammation, oxidative stress, and arterial calcification play far more significant roles. While statins may lower cholesterol numbers on a lab report, they do not address the what’s actually driving cardiovascular disease.

Metabolic Health and the Heart

When I refer to metabolic health, I’m not just talking about certain biomarkers like a lipid panel, cholesterol levels, or blood sugar. Metabolic health is about how efficiently your cells produce energy (ATP), how well mitochondria function, and whether your body is operating in a state of steady, sustained energy—or merely surviving on stress hormones.

The thyroid is the master regulator of metabolism. The thyroid produces triiodothyronine (T3), the active form of thyroid hormone is one of the major regulators mitochondrial function in heart cells. T3 stimulates mitochondrial biogenesis, oxygen consumption, and ATP production, ensuring the heart has the energy it needs. When thyroid function declines, mitochondria weaken, ATP production slows, and cells, including those in the heart, start to struggle. This is not because it’s damaged, but because its energy supply is restricted. This can lead to sluggish circulation, high cholesterol (due to impaired clearance), arterial stiffness, and even heart failure.

Dr. Broda Barnes, a pioneering physician, was one of the first to recognize the link between low thyroid function and heart disease. He observed that patients with undetected hypothyroidism often developed hypertension, high cholesterol, and cardiovascular disease, even when conventional lab tests showed no obvious issues. His research suggested that many cases of heart disease weren’t caused by cholesterol alone, but rather by an undiagnosed deficiency in thyroid hormone. Over 30 years ago, I joined the board of the Broda Barnes Research Foundation. Dr. Barnes was a brilliant man whose work on thyroid health was decades ahead of its time. I still recommend his book to all my patients, it’s a must read. 

Contemporary research has since proven Dr. Barnes findings. A comprehensive review published in the Journal of the American Heart Association found that overt hypothyroidism is associated with an increased risk of atherosclerotic cardiovascular disease. The study highlighted that thyroid hormone receptors are present in heart tissue, and that thyroid hormone deficiency can lead to adverse metabolic effects, including dyslipidemia, hypertension, and impaired cell function.

Patients with elevated thyroid-stimulating hormone (TSH) levels (the hormone that increases when thyroid function is low), even within the subclinical range, may have an increased risk of cardiovascular events. A study published in JAMA Network Open found that subclinical hypothyroidism was associated with a higher incidence of cardiovascular disease and all-cause mortality, particularly in those with TSH levels exceeding 7 mIU/L.

Despite Barnes’s work, the thyroid-heart connection remains largely overlooked in mainstream medicine. When a patient is at high risk for a cardiovascular event, or has high cholesterol, they are almost never recommended thyroid hormones.

Testing Thyroid Levels: 🦋

Cholesterol levels are considered the primary marker of cardiovascular health, but research suggests they are not a reliable predictor of heart disease risk at all.

In fact, a study of over 140,000 heart attack patients found that their LDL ("bad cholesterol") levels were lower than the recommended range at the time of admission to the hospital, suggesting that lowering LDL does not prevents heart attacks.

Instead of focusing solely on cholesterol, a most effective approach to heart health is addressing and monitoring thyroid function. When thyroid levels are low, cholesterol may build up rather than being efficiently used, which can influence both hormonal balance and cardiovascular function. High cholesterol is actually just a symptom of an underlying thyroid deficiency, not the true cause of heart disease.

To test thyroid levels, Dr. Barnes promoted the use of basal body temperature (BBT) as a reliable marker of thyroid function which can often prove more insightful than standard blood tests.

While TSH, T3, and T4 levels measure thyroid hormone circulation, they don't always reflect how well cells are using those hormones. Factors like PUFAs, high estrogen, low progesterone, and mineral imbalances (such as potassium or magnesium deficiencies) can block thyroid hormone uptake, leading to low metabolic function despite normal lab results. It’s also easy to measure at home and does not require a physician to order labs.

Barnes found that a consistently low morning temperature (below ~97.8°F/36.5°C) often indicated hypothyroidism, even when blood tests appeared normal, making BBT a more reliable tool for detecting cellular hypothyroidism.

How to Measure Basal Body Temperature (BBT) for Thyroid Function

1. Use a digital basal thermometer for accuracy. Avoid infrared or forehead thermometers as they may not be precise enough.

2. Take your temperature immediately upon waking, before getting out of bed or moving around. Keep the thermometer by your bedside to minimize activity before measuring.

3. Place under the tongue with lips closed.

4. Record the temperature and keep a log for at least 5–10 days to see trends. Women should measure on days 2–4 of their cycle (BBT naturally rises after ovulation due to progesterone).

5. Results:

   • Normal thyroid function: 97.8°F – 98.2°F (36.5°C – 36.8°C)

   • Possible hypothyroidism: Below 97.8°F (36.5°C) consistently

   • Possible hyperthyroidism: Above 98.6°F (37°C) consistently.

If your temperature is consistently low and you experience fatigue, cold hands/feet, slow metabolism, or hair loss, it may indicate poor thyroid function despite normal blood tests. Addressing thyroid health and bringing the body temperature back into the normal range is one of the most protective things we can do for cardiovascular health.

Strategies for Cardiovascular Health: 🦋🫀

1. Avoid PUFAs. The rise of heart disease correlates closely to the increased consumption of polyunsaturated fatty acids (PUFAs) in the form of vegetable oils like soybean, corn, and canola. PUFAs are highly unstable, meaning they can cause oxidative stress and damage cells. They are incorporated into cellular membranes, making them weaker and preventing the proper uptake of thyroid hormones. This inhibits the cell from making adequate energy which can cause widespread dysfunction, especially in heart. Opting for saturated fats (coconut oil, butter, tallow) is best. 

2. Eat a pro-thyroid diet. Fruit, milk, grass-fed meat, cheese, seafood, gelatin, sea salt, potatoes, vegetables, coconut oil, maple syrup, honey, etc. These foods have been consumed for thousands of years, are nutrient dense, and support healthy thyroid hormone production.

3. Lower stress and avoid fasting. Fasting is a major stress on the body. When our blood sugar drops from not eating, stress hormones rise to stabilize levels. These stress hormones (cortisol & adrenaline) oppose thyroid hormones and prevent cells from making energy efficiently. Eating consistently throughout the day and finding ways to manage stress are essential for keeping these hormones at bay, maintaining thyroid hormone production, and heart health.

4. Optimize thyroid levels. Supplementing thyroid directly is one of the best ways to normalize thyroid hormone levels. I prefer a combination of T4 and T3, the active hormone. Raena’s Natural Desiccated Thyroid is an OTC option that provides T3 and T4, and thyroid hormone precursors T1 and T2.

5. Optimize progesterone levels. Hormone imbalance, especially excess estrogen is linked to cardiovascular problems. Excess estrogen is linked to blood clotting, stroke, and heart attack, while progesterone is actualyl cardioprotective. Supplementing with bioidentical progesterone helps counter the effects of estrogen and promote healthy heart function. My go to:  Raena’s Natural Progesterone Cream and Progesterone Capsules.

6. Ensure adequate dietary calcium and along with vitamin K2. Contrary to popular belief, we need dietary calcium to prevent calcification. If we don’t get enough from diet, the body has to pull calcium from our bones and teeth, increasing the chances that it deposits in soft tissues causing calcification. Vitamin K2 is a powerful nutrient that ensures that calcium is deposited into bones and teeth, so it’s important to ensure adequate intake when you are eating a lot of calcium.

7. Use topical magnesium. Higher magnesium intake is consistently associated with lower risk of cardiovascular events. It soothes the heart muscle and promotes a healthy heart rate. Using topical magnesium before bed can support adequate magnesium levels, lower stress, and promote sleep. 

Resources: ❤️‍🩹

Dr. Broda Barnes - Hypothyroidism: The Unsuspected Illness

Dr. Ray Peat - Heart and Hormones

My recent article on statins and heart health: The Statin Paradox, Heart Disease, and Potential Solutions

Raena’s post on progesterone and the heart: How Does Progesterone Protect the Heart?

Talk soon ❤️,

Dr. C