Thyroxine – How the body Controls its Production

The thyroid gland can be taken to illustrate the basic principles of hormonal communication and action. Situated in the neck, the thyroid gland secretes thyroxine, a complex organic compound containing iodine. The iodine required for its synthesis is obtained from the diet. The structure of the thyroid gland  demonstrates the basic requirement of any endocrine organs, namely a close association between the secretary cells and the bloodstream. The secretary cells are arranged round a series of hollow follicles. Iodine is taken up into the cells from the bloodstream by active transport. The follicles contain an inactive precursor of the hormone, thyroglobulin, which is thyroxine conjugated with a protein. A proteeolytic enzyme secreted by the follicle epithelium separates thyroxine from the protein. The free hormone then passes through the wall of the follicle into the bloodstream. We shall now take a closer look at the control of the production of thyroxine.

Control of Thyroxine Production

How is the secretion of thyroxine kept to the requirements of the body? This is achieved by a negative feedback process of the kind discussed in Chapter 13. The shedding of thyroxine into the bloodstream is triggered by a hormone secreted by the anterior lobe of the pituitary gland 

This is called thyroid – stimulating or thyrotrophic hormone. Now the production of thyrotrophic hormone is regulated by thyroxine itself: a slight excess of thyroxine inhibits the anterior lobe of the pituitary which response by secreting less thyrotrophic hormone. This in turn reduces the activity of the thyroid gland, leading to a drop in the amount of thyroxine produced. This then removes the inhibitory influence on the pituitary so that more thyrotrophic hormone will be produced again, and so on. This is a good example of homeostasis and negative feedback 

Thyroxine is responsible for controlling the basal metabolic rate, and is therefore particularly important in growth. Under secretion of it during development (hypothyroidism) causes arrested physical and mental development, a condition called cretinism. A cretin aged 14 or 15 is stunted and pot-bellied and so mentally retarded that he may not even be able to feed himself. In adults the situation is not quite so serious since growth by this time is complete. The condition is called myxoedema, the symptoms being a decreased metabolic rate, increase in the amount of subcutaneous fat, coarsening of the skin and general physical and mental sluggishness.

Over – production of thyroxine (hyoperthyroidism) leads to a condition called exophthalmic goiter, so – called because of the characteristic swelling of the thyroid and protrusion of the eyeballs that accompanies it. Other symptoms include a greatly increased metabolic rate, loss of weight and accelerated heart beat, all this being accompanied by a general physical and mental restlessness. Plainly the thyroid is important in determining its owner’s general mental and emotional state, and it is interesting to speculate to what extent our own personalities may be altered by minor fluctuation in thyroid activity.

A deficiency of thyroxine in the body may be caused either by the thyroid gland failing to function properly, or to deficiency of iodine in the diet. The latter is Switzerland and the Great Lakes in the USA. In such areas the problem is overcome by adding iodine to the drinking water or supplying the inhabitants with iodized salt. The remedy for myxoedema used to be to take thyroid orally, the old prescription was one fried sheep’s thyroid weekly with black – currant jelly. Nowadays carefully regulated quantities of thyroxine are administered to the patient.

An over – abundance of thyroxine is generally caused by an over – active often excessively large, thyroid gland. In the old days the remedy was to remove a chunk of the thyroid, and hope for the best. If severe myxoedema resulted, small pieces of the thyroid were grafted back. Nowadays surgical removal of thyroid tissue is carried out with much greater precision. Some times surgery is avoided altogether by injection controlled doses of radio active iodine into the patient’s bloodstream; this is taken up by the thyroid cells, killing those in which it accumulates above a certain level.

What is Thyroxine?

Thyroxine is the primary hormone secreted by the thyroid gland and controls body metabolism. It also regulates the heart and digestive tract functions. Thyroxine contains levothyroxine as an active ingredient.

Thyroid hormones control your metabolism, heartbeat, body temperature, and muscle strength. They even affect your mood and growth. A blood test measures your thyroxine (T4) levels to diagnose thyroid disorders.

Functions of the Thyroid Gland

The thyroid is a butterfly-shaped gland in your neck, just in front of the windpipe (trachea). It makes hormones that are important for your body’s metabolism, growth and development. It also plays a critical role in controlling your heart, muscle and digestive function. In infants, it helps the brain and bones develop. The thyroid needs a steady supply of iodine to make its hormones.

The gland secretes two types of thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Neither hormone acts by itself; they work in combination with other proteins to regulate your body’s metabolism. Most of the thyroid hormone circulating in your blood is bound to protein molecules; only a small fraction, called free T4 or free T3, is able to enter tissues and exert its biological effect.

When the thyroid is working normally, it produces just the right amount of hormone to balance your metabolism and maintain normal bodily functions. But sometimes, the thyroid gets a little confused and starts to make too much or too little hormone. This is why it’s important to know the symptoms of thyroid dysfunction and see your St. Joseph and Texas A&M Health Network primary care physician if you experience them.

Insufficient thyroid hormone production is known as hypothyroidism. Symptoms include weakness, weight gain, sluggish metabolism and depression. In some cases, the condition is caused by Hashimoto’s disease, a disorder in which your immune system mistakenly attacks your thyroid. Too much thyroid hormone in the body is called hyperthyroidism. Symptoms can include irritability, restless sleep, sweating, extreme hunger, fast and erratic heartbeat (tachycardia) and changes in menstrual periods (amenorrhea).

The hormones produced by the thyroid affect your nervous system, causing symptoms like numbness or tingling. They also play a critical role in the reproductive and digestive systems, including the speed at which food moves through your digestive tract (gastrointestinal motility) and the occurrence of irregular or heavy menstrual periods. In some cases, an overactive thyroid can cause a fatty, goitre-like lump in the neck.

Weight Gain

Thyroid hormones regulate the body’s metabolism, digestive system, heart and muscle function, brain development, and bone formation. If you’re gaining weight, your thyroid may not be working well enough. A doctor will usually order a thyroid stimulating hormone (TSH) test to check how your thyroid is functioning. Then, if your TSH is normal, he or she may decide to do a thyroxine (T4) blood test to see how your thyroid gland is functioning.

A T4 test measures how much thyroxine is in your bloodstream, which is a precursor to the more active thyroid hormone triiodothyronine. Once released into the bloodstream, thyroxine is converted to triiodothyronine in tissue cells such as the liver, kidneys, and skeletal muscles. The thyroid gland also releases triiodothyronine into the bloodstream to help control the metabolism.

When the thyroid gland is healthy, it produces just the right amount of thyroxine and triiodothyronine for your needs. This is called the “normal range.” If your thyroid gland doesn’t produce enough thyroxine and/or produces too much thyroxine, it’s a problem. This condition is called hypothyroidism or underactive thyroid.

The hypothalamus in the brain secretes thyrotropin-releasing hormone, which stimulates the pituitary gland to produce thyroid stimulating hormone (TSH). When TSH levels are normal, the thyroid produces enough thyroxine and triiodothyronine to maintain your weight, eat, and grow at the rate that is appropriate for you.

If the thyroid gland doesn’t produce enough thyroxine, you might experience symptoms of an underactive thyroid such as fatigue, weakness, sluggish metabolism, constipation, and slowed heart rate. The condition can lead to weight gain, irregular menstrual cycle, irritability, and depression. In more severe cases, the thyroid can produce too much thyroxine and cause symptoms such as intolerance to heat, sweating, increased appetite, rapid heartbeat, and hair loss or thinning.

The best way to tell whether your thyroid isn’t working well is to get a T4 test. A health care professional will draw a blood sample from a vein in your arm. It should take less than five minutes to complete the test. You don’t need to do anything to prepare for the test except to let your health care provider know what medicines you are taking.


Fatigue is a feeling of extreme tiredness or lack of energy. It is different from sleepiness or drowsiness, which describe the need for rest or a desire to fall asleep. Fatigue can affect both physical and mental activities. It can interfere with work, school, home, and relationships. It can also make it difficult to feel happy.

Most people experience fatigue occasionally, such as after a long day or when they are recovering from illness. However, chronic fatigue can be a sign of an underlying medical condition. Getting a proper diagnosis and treatment can help alleviate the symptoms of fatigue.

Fatigue can be caused by a number of factors, including physical, emotional, and environmental stressors. It can also be a side effect of certain medications, such as corticosteroids and antidepressants. It can also be a result of some lifestyle habits, such as poor diet, excess caffeine or alcohol use, and irregular sleep patterns.

There are no specific tests to diagnose fatigue, but a doctor can examine the patient for signs of an underlying illness. They will ask the patient about their recent health history, any medications they are taking, and their sleep and exercise routines. They may also order blood tests to check for infections, thyroid disorders, anemia, liver disease, or diabetes. In some cases, they may also order an imaging scan to check for problems with the brain or bones.

There are some things you can do to help reduce your feelings of fatigue, such as getting enough sleep and reducing your stress levels. In addition, you can try eating a well-balanced diet and exercising regularly. You can also talk to your healthcare provider about whether changing your medication or adding other treatments, such as acupuncture or massage, might help relieve your symptoms. However, if you continue to feel tired despite these changes, it is important to see your doctor for a proper diagnosis and treatment. Left untreated, fatigue can lead to a variety of health complications.

Hair Loss

Hair loss can be a sign of thyroid problems. When the thyroid gland isn’t working correctly, your body may produce too little thyroxine. This can cause a condition called hypothyroidism. The hormones thyroxine and triiodothyronine are produced by the thyroid gland in your neck. They’re made in a feedback loop that involves the hypothalamus and pituitary gland. The hypothalamus secretes a hormone called thyrotropin-releasing hormone (TRH). TRH stimulates the pituitary gland to produce thyroid stimulating hormone (TSH). TSH then tells the thyroid to make thyroxine and triiodothyronine.

If you’re taking a medication that can affect the thyroid, such as steroids or some cancer drugs, it may reduce your production of thyroxine. Your doctor can recommend ways to adjust your doses and help you avoid any side effects.

A T4 test measures the amount of thyroxine in your blood. The results can tell your doctor if you have too much or too little of the thyroid hormone.

The blood sample for a T4 test is drawn from a vein in your arm. The health care professional cleans the skin at the puncture site and applies an elastic band to your upper arm to apply pressure and restrict blood flow through the vein. This causes the vein below the band to swell with blood. The blood is then withdrawn through the needle into a test tube or vial.

Your doctor can use a T4 test alone or in combination with a TSH (thyroid stimulating hormone) test to diagnose thyroid problems. Your doctor may also order a T4 test as part of a panel of tests that includes a TSH, T3 (triiodothyronine), and other thyroid tests to see if your thyroid is functioning properly.