11 Eye-Opening Facts About the Thyroid


The human body is an amazing thing. For each one of us, it’s the most intimate object we know. And yet most of us don’t know enough about it: its features, functions, quirks, and mysteries. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

The thyroid is a small, butterfly-shaped gland that lives just below your larynx. Its two halves, or lobes, which rest against the trachea, weigh less than an ounce. The thyroid is under the control a peanut-shaped gland in the brain called the pituitary gland, which in turn takes its commands from the hypothalamus, a region of the brain that works as the communications center for the pituitary—sending messages in the form of hormones to control the release of thyroid hormones from the pituitary.

Once stimulated, the thyroid gland takes up iodide from the foods we eat and converts it into iodine to make the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones are then released into the bloodstream, where they help your body regulate so many processes it would take several pages to describe them all. Generally, these hormones dictate your metabolism, digestion, fertility, weight loss, aging, and more.

Mental Floss spoke to a few experts to better understand this small but powerful gland. Here are 11 things we learned.


Thyroid hormones regulate the metabolic functions of literally every cell in the body by stimulating nearly all tissues in the body to produce proteins and by increasing oxygen available to cells.


You can think of your thyroid gland as your body’s furnace, and your pituitary gland as its thermostat, says Michelle Corey, a functional medical practitioner and author of The Thyroid Cure: The Functional Mind-Body Approach to Reversing Your Autoimmune Condition and Reclaiming Your Health! When the furnace (thyroid) gets too cold, the thermostat (pituitary) senses it and produces TSH to stimulate thyroid hormone production, which warms you up. When the levels of thyroid hormones rise and the furnace gets too hot, the pituitary gland then slows the production of TSH, cooling you down.


The liver is the major location where T4 is converted into the active T3. If your liver is taxed for any reason, it can’t do the job of converting these hormones, and you won’t have enough of the active thyroid hormone circulating in your body. As a result, you’ll feel sick and tired, even if you’re taking T4 hormone replacement. “If you have been diagnosed with an autoimmune thyroid condition, such as Hashimoto’s disease, supporting your liver is critical to recovery,” Corey says.


Often, symptoms of thyroid disorder may go unnoticed “since they are gradual and non-specific,” says Nilem Patel, an endocrinologist at Los Angeles’s Adventist Health White Memorial Hospital. “Left untreated, thyroid disorder can cause disruption in patients’ lives,” he says. Dysfunction in the thyroid can cause the thyroid to overproduce or underproduce thyroid hormones. If you suspect an issue with your thyroid, request tests beyond just your baseline TSH levels, including T3 and T4 levels as well as thyroid antibodies.


Your wee hour tossing and turning, as well as a racing heart and anxiety, could actually be symptoms of hyperthyroidism, or the overproduction of thyroid hormone. Other symptoms include fatigue, weight loss, palpitations, increased heart rate, and nervousness.


When these symptoms seemingly come out of nowhere, they can be evidence of an underproduction of thyroid hormone. Other common signs of a sluggish thyroid include fatigue, hair loss, constipation, dry skin, irregular menses, cold intolerance, brittle hair, slow heart rate, and general lethargy.


“The thyroid is the only gland to take up and trap iodine,” says Alan P. Farwell, section chief of endocrinology, diabetes, and nutrition at Boston Medical Center.

Thyroid hormones are also the only iodine-containing hormones. The thyroid gland not only takes up this element from dietary sources but stores a significant amount of iodinated tyrosines (a kind of amino acid) to maintain thyroid hormones in instances of iodine deficiency.


The gland can store a very large amount of hormone—so much that if the gland released all of its hormone into the bloodstream at once, it could kill you (this is known as thyrotoxicosis), says Linda Anegawa, a Hawaii-based physician with a specialty in obesity medicine. Fortunately thyroid hormone is very tightly regulated by constant, exquisitely sensitive signals traveling between the brain, the gland, the body’s tissues, and the blood concentrations of the hormone at any given moment.


To meet the increased metabolic needs of a pregnancy, a mother’s brain stimulates the thyroid gland to produce more hormone. “In the uterus, the fetal thyroid gland begins to function by 18 weeks of gestation. Should the fetus not get enough thyroid hormone from either the mother or from its own gland, severe outcomes can occur including abnormal brain development, abnormal growth of the skeletal system, problems with the placenta, or even miscarriage and increased perinatal mortality risk,” Anegawa says.


“I sometimes see patients with very elevated cholesterol on cholesterol-lowering medication that doesn’t seem to be working. But then I discover that their thyroid function is off-kilter,” says Anegawa. In these cases, she generally recommends adjusting a patient’s thyroid medicines or beginning treatment for at least six to eight weeks prior to checking the blood cholesterol level. This has helped some of her patients reduce their doses of cholesterol medicines, or stop taking them completely. “[Thyroid hormones] may someday be used as a cholesterol treatment, especially for patients who cannot tolerate statins, the most commonly used drugs,” she says.


A specially engineered form of thyroid hormone that only targets heart cells is under research as a treatment for heart failure, Anegawa says. Another form of the hormone, which selectively can enter nerve cells, may someday be a treatment for neurodegenerative disease.

7 Facts About Blood

Moussa81/iStock via Getty Images
Moussa81/iStock via Getty Images

Everyone knows that when you get cut, you bleed—a result of the constant movement of blood through our bodies. But do you know all of the functions the circulatory system actually performs? Here are some surprising facts about human blood—and a few cringe-worthy theories that preceded the modern scientific understanding of this vital fluid.

1. Doctors still use bloodletting and leeches to treat diseases.

Ancient peoples knew the circulatory system was important to overall health. That may be one reason for bloodletting, the practice of cutting people to “cure” everything from cancer to infections to mental illness. For the better part of two millennia, it persisted as one of the most common medical procedures.

Hippocrates believed that illness was caused by an imbalance of four “humors”—blood, phlegm, black bile, and yellow bile. For centuries, doctors believed balance could be restored by removing excess blood, often by bloodletting or leeches. It didn’t always go so well. George Washington, for example, died soon after his physician treated a sore throat with bloodletting and a series of other agonizing procedures.

By the mid-19th century, bloodletting was on its way out, but it hasn’t completely disappeared. Bloodletting is an effective treatment for some rare conditions like hemochromatosis, a hereditary condition causing your body to absorb too much iron.

Leeches have also made a comeback in medicine. We now know that leech saliva contains substances with anti-inflammatory, antibiotic, and anesthetic properties. It also contains hirudin, an enzyme that prevents clotting. It lets more oxygenated blood into the wound, reducing swelling and helping to rebuild tiny blood vessels so that it can heal faster. That’s why leeches are still sometimes used in treating certain circulatory diseases, arthritis, and skin grafting, and helps reattach fingers and toes. (Contrary to popular belief, even the blood-sucking variety of leech is not all that interested in human blood.)

2. Scientists didn't understand how blood circulation worked until the 17th century.

William Harvey, an English physician, is generally credited with discovering and demonstrating the mechanics of circulation, though his work developed out of the cumulative body of research on the subject over centuries.

The prevailing theory in Harvey’s time was that the lungs, not the heart, moved blood through the body. In part by dissecting living animals and studying their still-beating hearts, Harvey was able to describe how the heart pumped blood through the body and how blood returned to the heart. He also showed how valves in veins helped control the flow of blood through the body. Harvey was ridiculed by many of his contemporaries, but his theories were ultimately vindicated.

3. Blood types were discovered in the early 20th century.

Austrian physician Karl Landsteiner discovered different blood groups in 1901, after he noticed that blood mixed from people with different types would clot. His subsequent research classified types A, B and O. (Later research identified an additional type, AB). Blood types are differentiated by the kinds of antigens—molecules that provoke an immune system reaction—that attach to red blood cells.

People with Type A blood have only A antigens attached to their red cells but have B antigens in their plasma. In those with Type B blood, the location of the antigens is reversed. Type O blood has neither A nor B antigens on red cells, but both are present in the plasma. And finally, Type AB has both A and B antigens on red cells but neither in plasma. But wait, there’s more! When a third antigen, called the Rh factor, is present, the blood type is classified as positive. When Rh factor is absent, the blood type is negative.

Scientists still don’t understand why humans have different blood types, but knowing yours is important: Some people have life-threatening reactions if they receive a blood type during a transfusion that doesn’t “mix” with their own. Before researchers developed reliable ways to detect blood types, that tended to turn out badly for people receiving an incompatible human (or animal!) blood transfusion.

4. Blood makes up about 8 percent of our total body weight.

Adult bodies contain about 5 liters (5.3 quarts) of blood. An exception is pregnant women, whose bodies can produce about 50 percent more blood to nourish a fetus.)

Plasma, the liquid portion of blood, accounts for about 3 liters. It carries red and white blood cells and platelets, which deliver oxygen to our cells, fight disease, and repair damaged vessels. These cells are joined by electrolytes, antibodies, vitamins, proteins, and other nutrients required to maintain all the other cells in the body.

5. A healthy red blood cell lasts for roughly 120 days.

Red blood cells contain an important protein called hemoglobin that delivers oxygen to all the other cells in our bodies. It also carries carbon dioxide from those cells back to the lungs.

Red blood cells are produced in bone marrow, but not everyone produces healthy ones. People with sickle cell anemia, a hereditary condition, develop malformed red blood cells that get stuck in blood vessels. These blood cells last about 10 to 20 days, which leads to a chronic shortage of red blood cells, often causing to pain, infection, and organ damage.

6. Blood might play a role in treating Alzheimer's disease.

In 2014, research led by Stanford University scientists found that injecting the plasma of young mice into older mice improved memory and learning. Their findings follow years of experiments in which scientists surgically joined the circulatory systems of old and young mice to test whether young blood could reverse signs of aging. Those results showed rejuvenating effects of a particular blood protein on the organs of older mice.

The Stanford team’s findings that young blood had positive effects on mouse memory and learning sparked intense interest in whether it could eventually lead to new treatments for Alzheimer’s disease and other age-related conditions.

7. The sight of blood can make people faint.

For 3 to 4 percent of people, squeamishness associated with blood, injury, or invasive medical procedures like injections rises to the level of a true phobia called blood injury injection phobia (BII). And most sufferers share a common reaction: fainting.

Most phobias cause an increase in heart rate and blood pressure, and often muscle tension, shakes, and sweating: part of the body’s sympathetic nervous system’s “fight or flight” response. But sufferers of BII experience an added symptom. After initially increasing, their blood pressure and heart rate will abruptly drop.

This reaction is caused by the vagus nerve, which works to keep a steady heart rate, among other things. But the vagus nerve sometimes overdoes it, pushing blood pressure and heart rate too low. (You may have experienced this phenomenon if you’ve ever felt faint while hungry, dehydrated, startled, or standing up too fast.) For people with BII, the vasovagal response can happen at the mere sight or suggestion of blood, needles, or bodily injury, making even a routine medical or dental checkup cause for dread and embarrassment.

What Purpose Does the Belly Button Serve?

misuma/iStock via Getty Images
misuma/iStock via Getty Images

While your eyelashes are protecting your eyes, your lungs are letting you breathe, and virtually every other part of your body—inside and out—is performing its own relatively well-known task, your belly button is just sitting there collecting lint. And while it’s true that your navel served its most important purpose before you were born, it’s not totally useless now.

According to ZME Science, back when you were a fetus, your belly button was more of a belly portal: Your umbilical cord extended from it and connected you to the placenta on your mother’s uterine wall. That way, the placenta could channel nutrients and oxygen to you through the cord, and you could send back waste.

Your umbilical cord was cut when you were born, creating a tiny bulge that left behind some scar tissue after it healed. That scar tissue is your belly button, navel, or umbilicus. Though you may have heard that the shape of your belly button is a direct result of the scissor skills of the doctor who delivered you, that’s not true. Dr. Dan Polk, a neonatologist in the Chicago area, told the Chicago Tribune that a belly button's shape “has to do with how much baby skin leads onto the umbilical cord from the baby’s body. Less skin makes an innie; more skin makes an outie.” About 90 percent of people have innies.

Regardless of how your belly button looks, you probably don’t use it on a daily basis. However, if you’ve studied anatomy, medicine, or a related field, you might recognize it as the central point by which the abdomen is divided into the following quadrants: right upper, left upper, right lower, and left lower. Another way of classifying that area is into nine regions—including the hypochondriac, lumbar, iliac, epigastric, and hypogastric regions—with the umbilical region at the very center.

Abdominopelvic regions diagram
Blausen Medical, Wikimedia Commons // CC BY 3.0

Your belly button can also serve as the opening for laparoscopic surgery, which can save you from having a scar elsewhere on your abdomen.

The navel is a great central landmark outside of medicine, too. If you’ve taken yoga or Pilates classes, you may have heard it referred to as the center of balance or center of gravity. Because it sits right on top of your abdominal muscles, your belly button is an easy marker for your instructor to mention when they want you to access your core, which helps you balance.

And, of course, belly buttons are notorious for storing quite a bit of lint, which always seems to be blue (you can learn more about that here).

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.