The thyroid gland’s main function is to regulate your metabolism. It produces hormones that help control your heart and digestive system. In infants, thyroid hormones help the brain and bones develop.

A normal functioning thyroid gland is critical to the development of children and to both the long-term and minute-to-minute well-being of adults.

Thyroid Hormones

The thyroid is a butterfly-shaped organ located at the base of the neck in front of the trachea (windpipe). Measuring about 2 inches wide and weighing 20 to 60 grams, the thyroid’s function is to make hormones that are vitally important to the body’s metabolism, growth, and development.

The thyroid produces and releases two major hormones—thyroxine (T4) and triiodothyronine (T3). These hormones help to regulate many crucial bodily functions such as:

  • Heart rateBody weightMuscle strength and controlBreathingBody temperatureBone lossBlood lipid levelsMenstrual cyclesCentral nervous system (CNS)Energy expenditure

Iodine Factor

The distinguishing feature of these hormones is that they contain iodine atoms; T3 has three iodine atoms and T4 has four. Accordingly, the thyroid gland is unique in its specialized ability to pick up iodine from your bloodstream in order to incorporate it into your thyroid hormones.

Because iodine isn’t produced by the body naturally, it’s important to make sure you’re getting enough through your diet in order to keep your thyroid functioning well.

T4 Production

All T4 in your body is produced by the thyroid gland—about 80 to 100 micrograms (mcg) per day. Roughly 10 times that amount of T4, about 1000 mcg, is constantly circulating in your blood. More than 99 percent of the circulating T4 is bound to proteins in the plasma, chiefly, to thyroid-binding globulin (TBG), where it can be released if it’s needed. Only the small proportion of circulating T4 that is unbound, known as free T4, is immediately available for your body to use.

T4 Conversion Process

About 10 percent of the circulating T4, equivalent to the amount of new T4 released daily by your thyroid gland, is degraded each day. Generally, around half of this amount is converted to T3 by cleaving off one of its four iodine atoms. The remainder is converted to reverse T3 (rT3) by cleaving off an iodine atom from a different location.

The thyroid’s active hormone is T3, while rT3 is completely inactive.

T3 Production

Only about 20 percent of the T3 in your body, about 30 to 40 mcg per day, is produced by the thyroid gland. The other 80 percent is produced from T4 within the tissues, particularly by your kidneys, liver, muscle, brain, skin, and, when applicable, the placenta. T3 is degraded much more rapidly than T4.

What Happens in Your Body

Thyroid hormones have an impact on every cell and every organ. Specifically, T3 directly controls the production of various proteins made by your body’s cells. T3 does this by binding to a cell’s DNA.

Here’s a useful way to look at the thyroid hormones:

Free T4 and free T3 circulating in your blood are available to immediately enter your body’s cells whenever they’re needed; for instance, when you’re cold or when your body is digesting food. Some of the intracellular T4 is converted to T3, and some of the T3 binds to specific T3-receptors in the nucleus of the cell. This bound T3 causes nuclear DNA to stimulate (or inhibit) the production of specific proteins.

Among these proteins are various enzymes that, in turn, control the behavior of many important bodily functions mentioned above, such as how quickly your food is digested, your heart rate, body temperature, and how fast calories are burned.

Though thyroid hormones regulate DNA in this way in all cases, different cells in your body have different kinds of T3-nuclear receptors and in different concentrations. As such, the effect of T3 on a cell is quite variable from tissue to tissue and under various circumstances.

Thyroid System Regulation

Any time a physiological system is this critical, there are complex layers of regulation aimed at assuring that it is finely tuned to do what it needs to do and that its function is controlled within a narrow range. This is certainly true when it comes to the thyroid, which is part of the endocrine system.

Here’s a brief look at the major layers of thyroid regulation:

Pituitary-Thyroid Axis

The pituitary-thyroid axis provides the chief control over the thyroid gland itself. The pituitary gland, which is located deep within the brain, releases a thyroid stimulating hormone (TSH), causing the thyroid gland to increase its production and release of T3 and T4.

At the same time, circulating thyroid hormone, specifically T3, inhibits TSH production by the pituitary gland, thus forming a negative feedback loop. So, as T3 blood levels increase, TSH levels fall.

This feedback loop operates to keep the production of thyroid hormone by your thyroid gland within a narrow range.

Hypothalamus-Pituitary Axis

The pituitary gland is also prompted to release TSH when the hypothalamus releases thyrotropin-releasing hormones (TRH). The hypothalamus is a primitive part of the brain that coordinates many of your body’s basic functions and responds to numerous stimuli, including light and dark, smell, autonomic tone, several hormones, emotional stress, and neural inputs from the heart and gut.

When the hypothalamus releases TRH, this causes the pituitary gland to release more TSH, which in turn increases thyroid hormone production. Thus, thyroid hormone production is dependent on both TSH and on what the hypothalamus is “thinking and feeling” about the overall condition of your body and the environment.

Protein Binding

As mentioned previously, over 99 percent of the thyroid hormone in your bloodstream is bound to proteins in your blood, chiefly TBG, rendering the hormone inactive. Only free T4 and T3 have any physiologic activity. This protein binding of the thyroid hormones serves several critical regulatory functions:

  • It provides a large reservoir of circulating T4 to protect you in case your thyroid gland suddenly becomes less active. If this T4 reservoir was unavailable, your body tissues would be deprived of thyroid hormone within a few hours if your thyroid gland were to become temporarily nonfunctional.It maintains critical concentrations of free T3 and T4 within very narrow limits.It protects against any sudden increase in circulating free T3 should your tissues rapidly increase their conversion of T4 to T3.

Intracellular Regulation

As we have seen, T3 and T4 do their important work inside of your cells. Their normal functioning within cells—including their transport across the cell membrane from the blood to the interior of the cells, the conversion of T4 to T3, the crossing of T3 into the cell’s nucleus, and the binding of T3 to DNA—is dependent on a myriad of regulatory and transport proteins inside the cells whose identities and characteristics are still being discovered.

Thyroid Disorders

Given the complexity of all of this, it probably isn’t hard to imagine that there’s a lot of opportunity for something to go wrong. Thyroid disorders can occur with diseases affecting the thyroid gland itself or with conditions affecting the hypothalamus, pituitary, or blood proteins, or even with disorders affecting the handling of thyroid hormones by various tissues of the body.

In summary, the thyroid system is regulated at many levels:

In general, disorders of the thyroid system tend to cause thyroid function to become either underactive (hypothyroid) or overactive (hyperthyroid). In addition to these general problems, the thyroid gland can become grossly enlarged, a condition called a goiter, and people can develop cancer of the thyroid.

Any of these conditions are potentially very serious.

Symptoms

The symptoms of thyroid disease can be quite variable from individual to individual. Common symptoms of hypothyroidism often include:

  • Dry skinReduced heart rateSluggishnessPuffinessSkin changesHair lossLethargyWeight gain

Common symptoms of hyperthyroidism include:

  • Elevated pulseDry eyesLight sensitivityInsomniaThinning hairWeaknessTremors

Diagnosis

Diagnosing a thyroid disorder requires a careful analysis of screening thyroid blood tests and additional testing if a thyroid condition is suspected. In diagnosing a thyroid disorder, assessing the pituitary-thyroid relationship is particularly critical. This can generally be done by measuring the amount of total T3 and free T4 in your blood, as well as your TSH blood levels.

In some cases, the proper interpretation of TSH levels can be tricky and controversial.

If your TSH levels are elevated and your free T4 levels are low, this likely indicates that your thyroid gland is not producing enough hormone and your pituitary gland is attempting to whip up its function, which may indicate hypothyroidism. However, in rare instances high TSH levels could also be related to a pituitary tumor.

If your TSH levels are low and your free T4 levels are high, it may mean that your thyroid gland is producing too much thyroid hormone, but low TSH levels could also signify pituitary disease. Your endocrinologist should be able to provide a comprehensive diagnosis for you and may recommend additional testing if necessary.

A Word From Verywell

The thyroid gland and the hormones it produces are critically important to human development and a healthy life. The critical nature of thyroid function is reflected in the complex mechanisms that nature has established for the regulation of thyroid hormones.

Because the thyroid system is so important to our everyday functioning, it’s crucial to properly diagnose and treat any problems that occur. If you have symptoms of either hypothyroidism or hyperthyroidism, be sure to tell your healthcare provider so you can be tested.