Thyroid Gland

Reviewed by: BD Editors


The thyroid gland is a gland in the neck which secretes metabolic hormones important to the growth of the human body. It specifically regulates the creation and use of energy through the release of hormones. This gland does not rely on a duct system, so it is an endocrine gland.


The thyroid gland is by far the largest gland in the neck. It is shaped in the form of a butterfly, and it contains two lobes (one on each side of the neck). The entire structure is about two inches long, lying just below the Adam’s apple.

Glands come in endocrine and exocrine varieties. Since the thyroid gland functions by releasing hormones directly into the bloodstream, it is considered a key part of our endocrine system. Exocrine glands use ducts to excrete substances. The thyroid gland works primarily by absorbing iodine from our diets and then using it to make thyroid hormones thyroxine (T4) and triiodothyronine (T3). The thyroid gland is able to store these hormones for later use, as they will be released as needed. These hormones are then able to navigate through the entire body via the bloodstream in order to reach their target cells.

Thyroid Gland Function

The thyroid gland has the main role in controlling our body’s metabolism. The simplest way to define metabolism is our body’s ability to convert food into energy. This “fuel” is burned at different rates depending on the person, which is why people are said to have a “fast” or “slow” metabolism. Furthermore, the thyroid will secrete the hormones that will regulate our vitals and maintain our internal homeostasis. Among the most common, primitive functions of the body it controls are our breathing and our heart rate.

Our weight is likewise monitored by the thyroid gland, which explains why patients with a compromised thyroid gland will have a weight that fluctuates drastically, as we will discuss in more detail later. Even our internal body temperature and our cholesterol levels will be finely tuned with the help of thyroid hormone release.

The wings or lobes of the thyroid gland have a singular function. Their function is to synthesize thyroid hormone. They are able to have wide-spanning effects that affect nearly all of the tissues in the body with the help of the endocrine passageway. At the cellular level, the thyroid hormones are able to increase cellular (metabolic) activity. This influences not only our metabolic rate but also has an effect on protein synthesis. This, of course, facilitates normal development as growth relies on continual protein creation.

Thyroid Gland Location

As discussed, the thyroid gland is located in front of our neck. More specifically, it lies in front of our trachea, or “windpipe”. When the structure of the thyroid gland is inspected more closely, it has a reddish-brown coloration. The color is due to the fact that the thyroid gland is highly innervated and has its blood supplied by the superior and inferior thyroid arteries and the external carotid artery. The two-lobed structure will be linked by a bridge called the isthmus that lies in the middle of the lobes.

Thyroid Gland Anatomy
Thyroid Gland Anatomy

The image depicts the thyroid gland and the surrounding tissues & bones.

The location of the thyroid gland is quite easy to visualize, as it is an area that is regularly inspected during doctor visits. Of course, the gland at its normal size will not be perceptible and only becomes noticeable when the gland is swollen. But before birth, however, the placement of thyroid differs. It will be located in the back of the developing tongue, meaning it will migrate to the front of the neck (its post-birth location) prior to birth. The amount it travels also matters as issues will arise from thyroids that migrate too little or too far from the ideal mark. An extreme example is a condition called lingual thyroid, which is when the thyroid did not travel and instead remained in the back of the tongue.

The thyroid gland is truly a stand out through its versatile functions.

Thyroid Gland Hormones

The thyroid gland primarily makes and releases T3 and T4 hormones and the levels at which either thyroxine (T4) or triiodothyronine (T3) are released can be modulated to slow things down or speed things up. Thyroid hormones are made with the iodine flowing in our blood after a meal, which is then integrated into the physical structure of the hormones.

The thyroid cells that make up the gland have a special trait of being highly absorbent to iodine. Every remaining cell in the body will rely on the thyroid gland to manage its metabolism. Regardless, the levels of T3 to T4 will be eighty to twenty percent in a normal, functional thyroid gland.


The hypothalamus and the pituitary gland are the main controls that regulate the actual thyroid’s activity. When T3 or T4 hormone levels become too low, the hypothalamus will respond by secreting TSH-releasing hormone (or TRH). TRH signals the pituitary gland to create more thyroid stimulating hormone (or TSH). The thyroid gland, in turn, will respond by making more thyroid hormone in a feedback loop. The levels will be finely tuned to maintain a balance of T3 to T4 hormones.

TSH release will have a direct impact on the hormones’ respective levels. The role of TSH can be summed up as a stimulus that will lead the thyroid gland to release more hormone. Abnormally high levels of TSH can indicate an underactive thyroid. This is a condition called hypothyroidism. The related symptoms of having T3 and T4 in excess are listed below:

  • Anxiety
  • Hair loss
  • Irritability
  • Hyperactivity
  • Hand trembling


On the other hand, when the T3 and T4 levels fall under the functional amounts, the body will undergo changes in the opposite direction. Chronically high thyroid hormone levels will lead to hyperthyroidism. The high T3 and T4 levels will signal the pituitary gland to release less TSH in the system. Common symptoms may include the following:

  • Insomnia
  • Fatigue and tiredness
  • Depression
  • Difficulty concentration
  • Muscle pain

Normal Levels of Thyroid Activity

In summary, a high TSH will strongly suggest that the thyroid is underactive, or not making the right amount of thyroid hormone to sustain the body’s functions. The reason the levels of TSH are so high is that the body is trying to make the thyroid gland produce hormones to compensate for its lacking. A low TSH, in contrast, will infer a very active thyroid gland that is making too much thyroid hormone. In this case, the pituitary gland will try to inhibit TSH release to stop the thyroid gland from making more of it.

There is some variance over the reference range of TSH levels in the blood, but when the test reveals a level 0.5 or below, it is a possible sign of hyperthyroidism. When levels are above the 3-5.0 range, there is a good chance of hypothyroidism. This scale is hotly contested in the medical community but is still a parameter used to diagnose thyroid disorders. In addition to a blood test, an iodine thyroid scan will reveal if the origin of the thyroid hormone imbalance is a single nodule or the entire thyroid gland.

Thyroid Gland Disease

There are various thyroid illnesses, but the most common distill down to a few categories of imbalance:

  • Higher or lower production of thyroid hormone
  • Abnormal gland growth
  • Lumps or nodules within the thyroid gland
  • Thyroid cancer

Below are several common thyroid disorders.


Goiter is an enlargement of the thyroid gland from a noncancerous origin. Most commonly, goiter will arise in patients who have an iodine deficiency in their diet. This, of course, is a bit more common in places in the world where foods are not rich enough in iodine, or in women over the age of forty who are likelier to develop a goiter. The symptoms of goiter include swelling or tightness of the neck, breathing and swallowing difficulties, hoarseness of the voice, and wheezing. Typically, these clinical signs will only show when a patient’s thyroid has grown large enough. Goiter will be treated either with radioactive iodine doses or surgery.

Hashimoto’s Disease

Hashimoto’s disease is another thyroid disorder that is the most common cause of hypothyroidism in the U.S. The illness arises when a patient’s own immune system mistakenly attacks its own thyroid gland. This will, of course, comprise its ability to make hormones. The most common signs are fatigue, mild weight gain, dry skin and hair, depression, pale skin, and an enlarged goiter. Its cure is yet unknown.

Grave’s Disease

Grave’s disease is the most common cause of hyperthyroidism and is the foil of Hashimoto’s disease in that it arises from the patient’s immune system attacking its own thyroid gland. In this case, it will lead it to make more thyroid hormone than normal. The resulting symptoms may include anxiety, fatigue, hand tremors, excessive sweating, trouble sleeping, diarrhea, goiter, and bulging eyes or vision impairment. This condition will be treated with beta blockers that will slow the heart rate, as well as anxiety and anti-thyroid medications along with radioactive iodine.

Thyroid Nodules

Lastly, thyroid nodules are growths that form on the thyroid gland and may result from either Hashimoto’s disease or a lack of iodine. The nodules are mostly benign but can also become cancerous. The many signs it displays may include a high heart rate, nervousness, tremors, weight loss, and a big appetite.


1. When thyroid hormone levels are low, the pituitary gland will release ______ TSH?


2. Which condition is marked by a low TSH?


3. High TSH will indicate which of the following?


4. Which body system is the thyroid gland most involved in?


5. If a person's thyroid gland is malfunctioning, which of the following consequences will be readily observed?



Cite This Article

MLAAPAChicago Editors. "Thyroid Gland." Biology Dictionary,, 30 Jul. 2017, Editors. (2017, July 30). Thyroid Gland. Retrieved from Editors. "Thyroid Gland." Biology Dictionary., July 30, 2017.

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