Thyroid hormones

Your body has a tiny part that helps you.

It makes things that give you energy. These things help you grow. They also help your brain grow.

This part needs iodine from your food. Iodine helps it work well. If you do not get enough, the part can get big. This can make you feel tired. It is a very busy little part!

Your body has a small part called the thyroid gland.

This gland makes two main things. We call these thyroid hormones. They are named T3 and T4.

These hormones help your body use power. They also help your bones and brain grow.

To make these hormones, your body needs iodine. You get iodine from your food. Inside the gland, a protein called thyroglobulin is used.

An enzyme called thyroid peroxidase helps add iodine to this protein. This creates T3 and T4. T4 is the main form in your blood. T4 is later turned into T3 inside your cells. T3 is much more active than T4.

If you do not get enough iodine, your thyroid can get big. This is called a goitre. Doctors can use medicine to help. One common medicine is called levothyroxine. It is a man-made version of T4. It is a very important medicine for many people.

Thyroid hormones are essential signals that help your body work correctly. These hormones are made by a small part of your body called the thyroid gland.

There are two main types of these hormones, named T3 and T4. They are responsible for regulating your metabolism, which is how your body uses energy.

These hormones act on almost every cell in your body to keep things balanced. They help with growing bones and helping your brain develop properly. They also help your body stay warm and manage how you use fats and sugars.

Making these hormones is a very specific thing that happens inside the thyroid.

First, the gland uses a protein called thyroglobulin. To make the hormones, the body must add iodine to this protein. You get iodine from the food you eat. An enzyme called thyroid peroxidase helps attach iodine atoms to the protein. This creates T4, which has four iodine atoms. It also creates T3, which has three iodine atoms.

After they are made, these hormones move into your blood to travel to your cells.

Scientists have spent a long time learning about these important chemicals. In 1915, an American chemist named Edward Calvin Kendall isolated thyroxine. Later, in 1926, Charles Robert Harington and George Barger successfully produced synthetic thyroxine. This was a big step for medicine. Today, many people use man-made versions of these hormones if their body does not make enough. One common medicine is called levothyroxine. It is a manufactured version of the T4 hormone. In 2020, it was the second most prescribed medicine in the United States.

There are many interesting facts about how these hormones behave in your body. In your blood, there is much more T4 than T3. The ratio of T4 to T3 is about 14 to 1. While T4 is the main form in your blood, it is not the most active. Your cells must turn T4 into T3 to make it work. T3 is three to four times more potent than T4.

Also, the body needs selenium to make T3. This is why eating foods with selenium is important for your health.

Understanding thyroid hormones helps us see how everything in the body is linked. If a person does not get enough iodine, their thyroid gland can grow too large. This condition is called a simple goitre. Doctors can treat hormone shortages with pills taken by mouth. These pills are very important for many people around the world. The World Health Organization even puts levothyroxine on its List of Essential Medicines.

This shows just how much we rely on these tiny signals to stay healthy.

Thyroid hormones are vital chemical messengers that regulate how the body uses energy. Produced by the thyroid gland, these hormones are primarily responsible for managing metabolism. Metabolism is the process by which cells use energetic compounds to function. The two main hormones are triiodothyronine, known as T3, and thyroxine, known as T4.

These hormones are tyrosine-based and contain iodine, which humans must get from their food. Because they influence energy expenditure and cell development, they are essential for nearly every cell in the human body.

The production of these hormones is a complex, step-by-step biological process. It begins in the thyroid follicular cells with a large protein called thyroglobulin.

First, a mechanism called iodide trapping occurs. A protein called the Na+/I− symporter pumps iodide ions into the cell. This process uses a sodium concentration gradient to move iodide against its own gradient. Once inside, the iodide moves into the follicle lumen via a transporter called pendrin. In this lumen, an enzyme called thyroid peroxidase (TPO) helps turn the iodide into reactive iodine.

Once the iodine is reactive, it attaches to specific parts of the thyroglobulin protein. These parts are called tyrosyl residues. There are approximately 100 to 120 of these residues on each thyroglobulin molecule. The TPO enzyme facilitates a process where these iodinated residues pair up. When two specific residues pair, they form T4, which contains four iodine atoms.

If the pairing involves a different configuration, it forms T3, which has three iodine atoms.

Finally, the cell pulls the iodinated thyroglobulin back inside through endocytosis. Enzymes called proteases then cleave the protein to release the active T3 and T4 into the blood. This release is believed to happen largely through monocarboxylate transporters, specifically MCT 8 and 10.

These hormones serve different roles once they enter the bloodstream. T4 is the major form of thyroid hormone found in the blood. It has a relatively long half-life of about one week. In humans, the ratio of T4 to T3 released into the blood is approximately 14:1. However, T4 is not the most active form. To become fully functional, T4 must be converted into T3 within the target cells. This conversion is performed by selenium-containing enzymes called deiodinases. Because these enzymes require selenium, dietary selenium is essential for producing active T3. T3 is three to four times more potent than T4.

Scientists have made significant discoveries regarding these molecules over the last century. In 1915, the American chemist Edward Calvin Kendall successfully isolated thyroxine. Later, in 1926, Charles Robert Harington and George Barger produced synthetic thyroxine. These breakthroughs allowed medicine to address hormone deficiencies. Today, if a person has hypothyroidism, meaning a hormone deficiency, they can take replacement medication. Levothyroxine is a manufactured version of T4. It is metabolized slowly, so patients usually only need to take it once a day. In 2020, levothyroxine was the second most commonly prescribed medication in the United States, with over 98 million prescriptions. It is even included on the World Health Organization's List of Essential Medicines.

The importance of these hormones is seen in how they affect many bodily systems. They increase the basal metabolic rate and affect how the body synthesizes proteins. They also help regulate the growth of long bones and the maturation of the nervous system. Furthermore, they increase the body's sensitivity to catecholamines, such as epinephrine, which is especially important during cold exposure. Thyroid hormones even play a role in heat generation. Interestingly, the body also produces thyronamines, such as T1a and T0a. These are processed from T3 through decarboxylation and deiodination. While their exact mechanisms are unknown, they may inhibit neuronal activity, which is important for the hibernation cycles of mammals.

Maintaining the correct balance of iodine and thyroid function is critical for health. If a person lacks enough iodine, the thyroid cannot produce enough T3 and T4. This deficiency can cause the thyroid tissue to enlarge, creating a condition called a simple goitre. Proper hormone levels are also vital during pregnancy. If a pregnant woman has an untreated low-functioning thyroid, her fetus faces a higher risk of congenital disabilities. Because of this, doctors often must increase hormone dosages during pregnancy. While these hormones are generally well tolerated, they must be managed carefully in older patients to avoid risks to the heart. Understanding these tiny molecules helps us understand the very engine of human life.