Table of ContentsView AllTable of ContentsHormonesRegulationThyroid Disorders
Table of ContentsView All
View All
Table of Contents
Hormones
Regulation
Thyroid Disorders
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.
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Thyroid Hormones
The thyroid is a butterfly-shaped organ located at the base of the neck in front of thetrachea(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)andtriiodothyronine (T3). These hormones help to regulate many crucial bodily functions such as:
Verywell / Jessica Olah
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 itsspecialized ability to pick up iodinefrom your bloodstream in order to incorporate it into your thyroid hormones.
T4 Production
T4 Conversion Process
The thyroid’s active hormone is T3, while rT3 is completely inactive.
T3 Production
Here’s a useful way to look at the thyroid hormones:
What Happens in Your Body
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 athyroid stimulating hormone (TSH), causing the thyroid gland to increase its production and release of T3 and T4.
Hypothalamus-Pituitary Axis
The pituitary gland is also prompted to release TSH when the hypothalamus releasesthyrotropin-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.
Hormone release from the hypothalamus.
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 thethyroid hormone in your bloodstreamis 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:
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.
In summary, the thyroid system is regulated at many levels:
In general, disorders of the thyroid system tend to cause thyroid function to become eitherunderactive (hypothyroid)oroveractive (hyperthyroid). In addition to these general problems, the thyroid gland can become grossly enlarged, a condition called agoiter, and people can developcancer of the thyroid.
Any of these conditions are potentially very serious.
Symptoms
Thesymptoms of thyroid diseasecan be quite variable from individual to individual. Common symptoms ofhypothyroidismoften include:
Common symptoms ofhyperthyroidisminclude:
Diagnosis
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.
What Do High and Low TSH Levels Mean?
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.
14 Sources
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