Help Your Heart With Thyroid Hormones
The condition of low thyroid affects heart function as well as many other aspects of our health
By Heather S. Oliff, Ph.D.

side from that incredible hot fudge sundae you had last night, you try to eat healthy and exercise regularly. You know that it's important to keep your heart in top shape so you can stay fit and young. But did you know that if you are over 40 years old and have a poorly functioning thyroid gland - a condition that can sneak up on you so subtly that you're barely aware of it - there's more that you could be doing to decrease your risk for heart disease?

Hypothyroidism is a condition in which the thyroid, a small endocrine gland located in the front of the neck, fails to make enough thyroid hormones, of which there are four. These hormones are vital in controlling many aspects of metabolism - the myriad processes necessary for sustaining life, including the conversion of food into energy and heat.

The two most important thyroid hormones are thyroxine (also known as T4) and triiodothyronine (also known as T3); T4 is produced in the highest quantities, and some of it is converted in the body to T3, the most physiologically active of these compounds. The roles of the other two thyroid hormones, called T2 and T1, are not well understood, but we can assume that they must be important for our health - otherwise why would they be produced? There's a reason for everything, even if we don't know exactly what it is.

What Is Subclinical Hypothyroidism?

Overt hypothyroidism, as the name implies, is easily diagnosed, because the lab results are unequivocal: the thyroid hormone levels are below the range considered normal. But what if the thyroid is functioning poorly enough to cause symptoms, yet just well enough that the lab results are still within the normal range? This condition - often overlooked by doctors who place too much faith in lab tests alone - is a mild form of thyroid failure called subclinical hypothyroidism, or "low thyroid." It is an age-related condition that becomes apparent in the 40s, when all the functions of our endocrine glands begin to slow down.

There are three problems with subclinical hypothyroidism: (1) it can be hard to diagnose; (2) it's bad for you; and (3) if left untreated, it can progress to overt hypothyroidism, which is really bad for you. Every year, in fact, up to 20% of patients with subclinical hypothyroidism graduate to the real thing, which carries the potential for shortening their lives. The good news, though, is that hypothyroidism, whether subclinical or overt, is easy to treat: one takes thyroid hormones (often for life) to correct the deficiency.

Symptoms of Hypothyroidism

Subclinical hypothyroidism manifests itself in so many ways that it can be hard to diagnose, especially as many of the symptoms are associated with the aging process and are thus easily written off. Following are most of the symptoms, only a few of which are likely to appear in any given case of the disease:

  • Fatigue
  • Constipation
  • Weakness, lethargy
  • Dry skin
  • Feeling cold
  • Cold skin
  • Cold intolerance
  • Coarse, dry hair
  • Weight gain
  • Muscle cramps
  • Mild eyelid, face, or leg swelling
  • Hair loss
  • Brittle, split nails
  • Slow heart rate
  • Hoarseness
  • Hearing loss
  • Yellow tint to skin
  • Decrease or loss of sweating
  • Pale lips or skin
  • Memory loss

How Is Low Thyroid Diagnosed?

With thyroid hormones, the trick lies in knowing when to start taking them - i.e., when does the condition of low thyroid actually exist? This can be difficult to determine, because there are many different symptoms (see the sidebar), and many of them are more annoying than threatening, so they can be easily dismissed as relatively inconsequential. Physicians and patients alike often chalk them up to "normal aging" and resign themselves to being unable to do anything about them.

A sensitive physician - one who is attuned to the patient's whole person, not just one part of his or her body - will likely pick up on the symptoms and recognize what they mean. Count yourself lucky if you have such a physician. And if you happen to have low thyroid, you are probably a woman, because this disorder is more common in women than in men (life is unfair). According to some estimates, as many as 15-20% of women over the age of 60 may be affected. People with a family history or personal history of thyroid disease are more prone to develop low thyroid, and smoking may worsen it (as it worsens just about everything).

Low Thyroid Increases the Risk for Heart Disease

In the large-scale Rotterdam chronic disease study in the Netherlands, researchers found that 11% of a randomly selected group of 1149 postmenopausal women (average age 69 years) had subclinical hypothyroidism.1 Interestingly - and ominously - it was found that these same women were almost twice as likely as those without the thyroid condition to have blockages in their aorta (the major artery from the heart), and they were twice as likely to have had a heart attack.

Clearly, thyroid hormones are important for protecting the cardiovascular system, whose proper functioning is vital for delivering enough oxygen and other nutrients to the body. We know that the thyroid hormones help control heart rate and blood pressure. With low thyroid, the heart rate can slow down, and arrhythmias (abnormal or irregular heart beats) can develop. Also, blood pressure may fall, and cardiac output (the volume of blood pumped by the heart with each stroke) declines.

Thyroid Therapy Improves Heart Function

Hypothyroidism can cause changes in heart function during diastole, the period between beats, when the heart fills with blood. The problem is that the heart does not fill with enough blood, and this can lead to heart failure - an inability of the heart to pump blood efficiently enough to serve the body's needs for life-sustaining nutrients.

In hypothyroid patients, however, diastolic dysfunction in the heart's left ventricle - the chamber that pumps blood into the aorta - may be reversible with thyroid hormone treatment. In a controlled clinical trial, researchers tested the effect of thyroid hormone therapy on heart function in 12 hypothyroid women.2 The women (average age 47) received thyroxine (T4) injections at an average dose of 136 micrograms per day for 1-2 months. Filling of the heart's left ventricle during diastole was measured by a reliable, noninvasive technique called Doppler echocardiography, in which ultrasonic radiation is used to create an image of the heart, similar to the way in which bats "see" objects in total darkness by sensing reflected ultrasound waves.

T3 increased the heart’s stroke
volume (it also increased the heart
rate slightly), and it caused the
blood vessels to dilate, allowing
blood to flow more freely.

The results showed a clear improvement in heart function when the patients' thyroxine levels returned to normal, and their average heart rate improved from 61 to 68 beats per minute.

Thyroid Hormones Improve Cardiac Output and Vascular Resistance

Hypothyroidism decreases cardiac output and increases systemic vascular resistance (resistance to blood flow throughout the circulatory system), both of which result in reduced delivery of blood to the tissues. Since it is known that triiodothyronine (T3) is important in cardiovascular function, researchers wanted to see if it could improve cardiac output and vascular resistance, even in healthy individuals.

In a randomized, placebo-controlled, double-blind study, 18 healthy men (average age 26) were injected with 100 micrograms of T3 or placebo.3 Cardiac output and vascular resistance were repeatedly measured for 45 minutes after the injection. The researchers found that T3 caused a statistically significant increase in cardiac output and a statistically significant decrease in vascular resistance, both within 3 minutes. Thus, T3 increased the heart's stroke volume (it also increased the heart rate slightly), and it caused the blood vessels to dilate, allowing blood to flow more freely.

The hormone injections in this study produced blood levels of T3 that were in the pharmacological, not physiological range, meaning that they exceeded the levels found in normal, healthy individuals. This, of course, is not the goal of conventional thyroid therapy, which aims to restore normal levels of the hormones in those who are deficient. Nonetheless, the results suggest that thyroid therapy may provide similar cardiovascular benefits - increased cardiac output and decreased vascular resistance - in people with low thyroid.

Low Thyroid May Mean Elevated Lipid Levels

The number one cause of high cholesterol is a poor diet: low in fiber and high in the wrong kinds of fat (the primary source of cholesterol). The second leading cause is hypothyroidism. When thyroid hormone levels drop, the liver no longer functions properly, and it produces excess cholesterol, fatty acids, and triglycerides (fats). These increase the risk for atherosclerosis, and thus heart disease.

There is a well-established relationship between overt hypothyroidism and serum lipid levels - elevated LDL - cholesterol ("bad cholesterol"), reduced HDL-cholesterol ("good cholesterol"), and elevated triglycerides. The relationship between subclinical hypothyroidism and serum lipid levels, however, is controversial. Some studies have reported correlations similar to those in overt hypothyroidism, whereas other studies have found no such correlations. This may be explained by all of the studies having different patient profiles with respect to such factors as age, smoking status, and range of serum T4 levels, as well as the inclusion (or not) of patients with an unstable form of the disorder.

Thyroid Hormones Reduce Cholesterol Levels

An Italian research group sought to resolve the controversy by rigorously selecting patients with subclinical hypothyroidism (of at least 6 months duration) and then verifying the potential benefit of T4 therapy.4 Forty-nine patients (7 men and 42 women, average age 35) and 33 normal individuals (sex- and age-matched) participated in a randomized, placebo-controlled study, i.e., they were randomly assigned to receive either T4 tablets or placebo. The T4 dosage was 25 micrograms per day initially, and it was then gradually increased to an average of 68 micrograms per day (the individual doses varied because each patient required a different amount of T4 to regain normal, stable levels). Lipid levels were evaluated at baseline (before treatment) and after 6 months of having had stable, normal thyroid hormone levels.

The results showed that at baseline, the patients with subclinical hypothyroidism had statistically higher serum total cholesterol and LDL levels than the controls. After 6 months of stable, normal T4 levels, the patients treated with T4 had a statistically significant decrease in serum total cholesterol and LDL levels, whereas the patients on placebo had not improved. Thus, by decreasing the levels of LDL (the bad cholesterol), T4 therapy may reduce the risk for atherosclerosis and heart disease.

As a side note, another study has found that smoking cigarettes increases serum concentrations of total and LDL cholesterol in women with subclinical hypothyroidism.5 Thus, smoking exacerbates the cardiovascular consequences of this disease.

Thyroid Hormones Offer Many Benefits

Treating subclinical hypothyroidism with thyroid hormones prevents the progression to overt hypothyroidism. Proper treatment alleviates the annoying symptoms of thyroid deficiency, which appear to be reversible in at least 25% of patients. Since many of the symptoms are associated with the aging process, alleviating them may make you feel younger and livelier, not to mention that it helps protect your heart.

By decreasing the levels of LDL
(the bad cholesterol), T4 therapy
may reduce the risk for
atherosclerosis and heart disease
in people with low thyroid.

If you have some of the symptoms of low thyroid but it hasn't occurred to you (or your doctor) that you may have low thyroid, you owe it to yourself to check out that possibility. And if you do have it, you may find that thyroid supplements can make you a healthier, happier person. Isn't that what we all want?


  1. Hak EA, Pols HAP, Visser TJ, Drexhage HA, Hofman A, Witteman JCM. Subclinical hypothyroidism is an independent risk factor for atherosclerosis and myocardial infarction in elderly women: the Rotterdam study. Ann Intern Med 2000;132:270-8.
  2. Virtanen VK, Saha HHT, Groundstroem KWE, Salmi J, Pasternack AI. Thyroid hormone substitution therapy rapidly enhances left-ventricular diastolic function in hypothyroid patients. Cardiology 2001;96:59-64.
  3. Schmidt BMW, Martin N, Georgens AC, Tillmann H-C, Feuring M, Christ M, Wehling M. Nongenomic cardiovascular effects of triiodothyronine in euthyroid male volunteers. J Clin Endocrin Metab 2002;87(4):1681-6.
  4. Caraccio N, Ferrannini E, Monzani F. Lipoprotein profile in subclinical hypothyroidism: response to levothyroxine replacement, a randomized placebo-controlled study. J Clin Endocrin Metab 2002;87(4):1533-8.
  5. Muller B, Zulewski H, Huber P, Ratcliffe JG, Staub JJ. Impaired action of thyroid hormone associated with smoking in women with hypothyroidism. N Engl J Med 1995;333(15):964-9.

Dr. Oliff is a pharmacologist and toxicologist with a specialty in neuroscience. She has experience as a research scientist and educator, and has been writing about nutritional supplements since 1998.

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