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.

25 Amazing Facts About the Human Body


The human body is an amazing piece of machinery—with a few weird quirks.

  1. It’s possible to brush your teeth too aggressively. Doing so can wear down enamel and make teeth sensitive to hot and cold foods.

  2. Goose bumps evolved to make our ancestors’ hair stand up, making them appear more threatening to predators.

Woman's legs with goosebumps
  1. Wisdom teeth serve no purpose. They’re left over from hundreds of thousands of years ago. As early humans’ brains grew bigger, it reduced space in the mouth, crowding out this third set of molars.

  2. Scientists aren't exactly sure why we yawn, but it may help regulate body temperature.

  3. Your fingernails don’t actually grow after you’re dead.

  4. If they were laid end to end, all of the blood vessels in the human body would encircle the Earth four times.

  5. Humans are the only animals with chins.

    An older woman's chin
    1. As you breathe, most of the air is going in and out of one nostril. Every few hours, the workload shifts to the other nostril.

    2. Blood makes up about 8 percent of your total body weight.

    3. The human nose can detect about 1 trillion smells.

    4. You have two kidneys, but only one is necessary to live.

    5. Belly buttons grow special hairs to catch lint.

      A woman putting her hands in a heart shape around her belly button
      1. The satisfying sound of cracking your knuckles comes from gas bubbles bursting in your joints.

      2. Skin is the body’s largest organ and can comprise 15 percent of a person’s total weight.

      3. Thumbs have their own pulse.

      4. Your tongue is made up of eight interwoven muscles, similar in structure to an elephant’s trunk or an octopus’s tentacle.

      5. On a genetic level, all human beings are more than 99 percent identical.

        Identical twin baby boys in striped shirts
        1. The foot is one of the most ticklish parts of the body.

        2. Extraocular muscles in the eye are the body’s fastest muscles. They allow both of your eyes to flick in the same direction in a single 50-millisecond movement.

        3. A surgical procedure called a selective amygdalohippocampectomy removes half of the brain’s amygdala—and with it, the patient’s sense of fear.

        4. The pineal gland, which secretes the hormone melatonin, got its name from its shape, which resembles a pine nut.

        5. Hair grows fast—about 6 inches per year. The only thing in the body that grows faster is bone marrow.

          An African-American woman drying her hair with a towel and laughing
          1. No one really knows what fingerprints are for, but they might help wick water away from our hands, prevent blisters, or improve touch.

          2. The heart beats more than 3 billion times in the average human lifespan.

          3. Blushing is caused by a rush of adrenaline.

12 Intriguing Facts About the Intestines

When we talk about the belly, gut, or bowels, what we're really talking about are the intestines—long, hollow, coiled tubes that comprise a major part of the digestive tract, running from the stomach to the anus. The intestines begin with the small intestine, divided into three parts whimsically named the duodenum, jejunum, and ileum, which absorb most of the nutrients from what we eat and drink. Food then moves into the large intestine, or colon, which absorbs water from the digested food and expels it into the rectum. That's when sensitive nerves in your rectum create the sensation of needing to poop.

These organs can be the source of intestinal pain, such as in irritable bowel syndrome, but they can also support microbes that are beneficial to your overall health. Here are some more facts about your intestines.

1. The intestines were named by medieval anatomists.

Medieval anatomists had a pretty good understanding of the physiology of the gut, and are the ones who gave the intestinal sections their names, which are still used today in modern anatomy. When they weren't moralizing about the organs, they got metaphorical about them. In 1535, the Spanish doctor Andrés Laguna noted that because the intestines "carry the chyle and all the excrement through the entire region of the stomach as if through the Ocean Sea," they could be likened to "those tall ships which as soon as they have crossed the ocean come to Rouen with their cargoes on their way to Paris but transfer their cargoes at Rouen into small boats for the last stage of the journey up the Seine."

2. Leonardo da Vinci believed the intestines helped you breathe.

Leonardo mistakenly believed the digestive system aided respiratory function. In 1490, he wrote in his unpublished notebooks, "The compressed intestines with the condensed air which is generated in them, thrust the diaphragm upwards; the diaphragm compresses the lungs and expresses the air." While that isn't anatomically accurate, it is true that the opening of the lungs is helped by the relaxation of stomach muscles, which does draw down the diaphragm.

3. Your intestines could cover two tennis courts ...

Your intestines take up a whole lot of square footage inside you. "The surface area of the intestines, if laid out flat, would cover two tennis courts," Colby Zaph, a professor of immunology in the department of biochemistry and molecular biology at Melbourne's Monash University, tells Mental Floss. The small intestine alone is about 20 feet long, and the large intestine about 5 feet long.

4. ... and they're pretty athletic.

The process of moving food through your intestines requires a wave-like pattern of muscular action, known as peristalsis, which you can see in action during surgery in this YouTube video.

5. Your intestines can fold like a telescope—but that's not something you want to happen.

Intussusception is the name of a condition where a part of your intestine folds in on itself, usually between the lower part of the small intestine and the beginning of the large intestine. It often presents as severe intestinal pain and requires immediate medical attention. It's very rare, and in children may be related to a viral infection. In adults, it's more commonly a symptom of an abnormal growth or polyp.

6. Intestines are very discriminating.

"The intestines have to discriminate between good things—food, water, vitamins, good bacteria—and bad things, such as infectious organisms like viruses, parasites and bad bacteria," Zaph says. Researchers don't entirely know how the intestines do this. Zaph says that while your intestines are designed to keep dangerous bacteria contained, infectious microbes can sometimes penetrate your immune system through your intestines.

7. The small intestine is covered in "fingers" ...

The lining of the small intestine is blanketed in tiny finger-like protrusions known as villi. These villi are then covered in even tinier protrusions called microvilli, which help capture food particles to absorb nutrients, and move food on to the large intestine.

8. ... And you can't live without it.

Your small intestine "is the sole point of food and water absorption," Zaph says. Without it, "you'd have to be fed through the blood."

9. The intestines house your microbiome. 

The microbiome is made up of all kinds of microorganisms, including bacteria, viruses, fungi, and protozoans, "and probably used to include worm parasites too," says Zaph. So in a way, he adds, "we are constantly infected with something, but it [can be] helpful, not harmful."

10. Intestines are sensitive to change.

Zaph says that many factors change the composition of the microbiome, including antibiotics, foods we eat, stress, and infections. But in general, most people's microbiomes return to a stable state after these events. "The microbiome composition is different between people and affected by diseases. But we still don't know whether the different microbiomes cause disease, or are a result in the development of disease," he says.

11. Transferring bacteria from one gut to another can transfer disease—or maybe cure it.

"Studies in mice show that transplanting microbes from obese mice can transfer obesity to thin mice," Zaph says. But transplanting microbes from healthy people into sick people can be a powerful treatment for some intestinal infections, like that of the bacteria Clostridium difficile, he adds. Research is pouring out on how the microbiome affects various diseases, including multiple sclerosis, Parkinson's, and even autism.

12. The microbes in your intestines might influence how you respond to medical treatments.

Some people don't respond to cancer drugs as effectively as others, Zaph says. "One reason is that different microbiomes can metabolize the drugs differently." This has huge ramifications for chemotherapy and new cancer treatments called checkpoint inhibitors. As scientists learn more about how different bacteria metabolize drugs, they could possibly improve how effective existing cancer treatments are.