Your thyroid gland produces the hormones T3 and thyroxine (T4), which play a role in metabolism. Your thyroid also produces calcitonin, which helps regulate blood calcium levels. In this lesson, you will learn about the thyroid gland and its hormones, including how they are regulated by TSH.

Thyroid Gland

The thyroid gland is wrapped around the windpipe.
Thyroid Gland Location

At the base of your neck, just below your Adam’s apple, lies a hormone-producing gland called the thyroid gland. In this lesson, you will learn about the anatomical features of this gland and the functions of the three hormones it produces: T3, T4, and calcitonin.

But, before we get started, let’s talk about the thyroid gland itself. I bet you have heard of the thyroid gland before. This gland is familiar to most people because it often gets the blame for a slow metabolism and weight gain. It is true that the thyroid gland plays an important role in your metabolic rate. It’s also true that an underactive thyroid gland can leave a person feeling tired and sluggish – and cause that person to experience weight gain. However, these symptoms can also result from an unhealthy lifestyle, and in many cases, the thyroid gland gets more blame than it deserves. So, let’s take a closer look at the thyroid gland and how it helps your body function properly.

Your thyroid gland is an endocrine gland that secretes hormones that control metabolism and body growth. As mentioned earlier, it is found at the base of your neck and wraps itself around your trachea, or windpipe. When you look at a drawing of the thyroid gland, it almost resembles a butterfly with its wings wide open sitting on a plant stem. The ‘wings’ are actually the two lobes of the thyroid gland. If you were to take a look inside these lobes, you would notice many hollow structures called thyroid follicles. These follicles contain a protein-rich fluid, from which we get the thyroid hormones T3 and T4.

T3 and Thyroxine (T4)

These thyroid hormones are somewhat unique because they need iodine in order to be produced. Therefore, you need to take in iodine for your thyroid gland to function properly. You get iodine from foods such as dairy products and seafood, and even if you avoid these foods, you likely get enough iodine in your diet due to the fact that salt is often iodized – meaning that it has iodine added. It is interesting to note that because the thyroid gland absorbs nearly all of the iodine in your body, iodine can be made into a radioactive isotope and given to a patient as radioactive iodine to assess the health of the thyroid.

Triiodothyronine, or T3, contains three atoms of iodine.
Triiodothyronine

In your body, iodine is combined with an amino acid, and from this combination we get the two thyroid hormones that are important to your metabolism. If the hormone has three iodine atoms attached to it, we call it T3, or triiodothyronine. This term is not easy to pronounce, but it is easy to recall if you break it down. For instance, we know that the prefix ‘tri-‘ means ‘three,’ and the term ‘iodo’ refers to ‘iodine.’ So, this hormone is composed of three iodine atoms. T3 is a powerful thyroid hormone that affects almost every process in the body, including metabolism, body temperature, growth and development, and heart rate.

In the same way we just learned about T3, if the hormone as four iodine atoms attached to it, we call it T4, or tetraiodothyronine. We see that the prefix ‘tetra-‘ means ‘four.’ However, T4 is not often referred to as tetraiodothyronine. It is far more common to refer to this hormone as T4 or thyroxine. You can recall that thyroxine is a thyroid hormone by noticing that both words start with the letters ‘t-h-y-r-o.’ Thyroxine is very similar to T3, and we see that T3 and T4 work together to control processes in the body, including metabolism, body temperature, growth and development, and heart rate.

TSH

As we are learning from studying the endocrine system, the hormonal output of one endocrine gland is often dependent on the hormones from another endocrine gland. This is the case when we look at the thyroid. Thyroid-stimulating hormone (TSH) is a hormone that stimulates the thyroid gland to produce T3 and T4. We previously learned that thyroid-stimulating hormone is secreted by the anterior pituitary gland, which is the important master gland that is responsible for affecting many body organs.

Of course, we have to be careful not to get too carried away with calling the anterior pituitary gland the ‘master gland’ because it would not be able to release TSH if it were not for some assistance from its important helper: the hypothalamus, which must first produce releasing hormones. So, we see that one endocrine organ depends on another, and this is almost like watching dominoes fall down. We see that releasing hormone from the hypothalamus causes the release of the hormone TSH from the anterior pituitary gland, which finally allows the release of the hormones T3 and T4 from the thyroid gland.

The hormone TSH is produced by the anterior pituitary gland.
Pituitary Gland Produces TSH

Calcitonin

So, we see that it’s pretty obvious that your thyroid gland gets the most attention for the role it plays in your metabolism, but we should not overlook the fact that there is another hormone produced in your thyroid gland that has nothing to do with your metabolism. This hormone is called calcitonin. It is defined as a thyroid-secreted hormone that lowers calcium levels in the blood. A good amount of the calcium that’s removed from your blood goes into your bones. You can recall that calcitonin affects calcium levels by noticing that both calcitonin and calcium start with the letters ‘c-a-l’ and remember that calcitonin puts calcium ‘in’ your bones. So, if there is excess calcium floating around your bloodstream, your thyroid gland produces and secretes calcitonin, which sweeps up the excess calcium and deposits it in your bones.

Later on, we will learn about the antagonist of calcitonin, called parathyroid hormone (PTH). This hormone does the opposite of calcitonin, so they are in a constant tug-of-war match with each other. Calcitonin is on one end of the rope acting to reduce the blood level of calcium and inhibit bone breakdown, and parathyroid hormone is on the other end of the rope acting to increase the blood calcium level and increase bone breakdown.

Lesson Summary

Let’s review. Your thyroid gland is an endocrine gland found at the base of your neck. The thyroid follicles found inside the gland contain a protein-rich fluid, from which we get the thyroid hormones T3 and T4.

Your diet must include iodine, which is easily obtained from iodized salt, in order to form the thyroid hormones. If there are three iodine atoms attached to the hormone, we call it T3, or triiodothyronine. If there are four iodine atoms attached, we call it T4, or thyroxine. T3 and T4 work together to control processes in the body, including metabolism, body temperature, growth and development, and heart rate.

Before these hormones can be formed, your anterior pituitary gland must secrete thyroid-stimulating hormone (TSH), which is a hormone that stimulates the thyroid gland to produce T3 and T4.

Besides influencing your metabolism, your thyroid gland also plays a role in regulating blood calcium levels through another hormone called calcitonin. Calcitonin has the effect of lowering blood calcium.

Learning Outcomes

After watching this video, you should be able to:

  • Describe the structure and function of the thyroid gland
  • Understand the importance of iodine in the production of T3 and T4
  • Explain the roles of T3, T4, TSH, and calcitonin