Chemical structure of thyroxine
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Liothyronine Sodium

The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are tyrosine-based hormones produced by the thyroid gland. An important component in the synthesis is iodine. The major form of thyroid hormone in the blood is thyroxine (T4). This is converted to the active T3 within cells by deiodinases. These are further processed by decarboxylation and deiodination to produce iodothyronamine (T1a) and thyronamine (T0a). more...

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Circulation

Most of the thyroid hormone circulating in the blood is bound to transport proteins :

  • Thyroxine-binding globulin (TBG)
  • Thyroid-binding prealbumin (TBPA) - this protein is also responsible for the transport of retinol, and so now has the preferred name of transthyretin (TTR)
  • albumin.

Only a very small fraction of the circulating hormone is free (unbound) - T4 0.03% and T3 0.3%. This free fraction is biologically active, hence measuring concentrations of free thyroid hormones is of great diagnostic value. These values are referred to as fT4 and fT3. Another critical diagnostic tool is the amount of thyroid-stimulating hormone that is present. When thyroid hormone is bound, it is not active, so the amount of free T3/T4 is what is important. For this reason, measuring total thyroxine in the blood can be misleading.

Function

The thyronines act on the body to increase the basal metabolic rate, affect protein synthesis and increase the body's sensitivity to catecholamines (such as adrenaline).The thyroid hormones are essential to proper development and differentiation of all cells of the human body. To various extents, they regulate protein, fat and carbohydrate metabolism. But they have their most pronounced effects on how human cells use energetic compounds. Numerous physiological and pathological stimuli influence thyroid hormone synthesis.

The thyronamines function via some unknown mechanism to inhbit neuronal activity; this plays an important role in the hibernation cycles of mammals. One effect of administering the thyronamines is a severe drop in body temperature.

Related diseases

Both excess and deficiency of thyroxine can cause disorders.

  • Thyrotoxicosis or hyperthyroidism is the clinical syndrome caused by an excess of circulating free thyroxine, free triiodothyronine, or both. It is a common disorder that affects approximately 2% of women and 0.2% of men.
  • Hypothyroidism is the case where there is a deficiency of thyroxine.

Medical use of thyroid hormones

Both T3 and T4 are used to treat thyroid hormone deficiency (hypothyroidism). They are both absorbed well by the gut, so can be given orally. Levothyroxine, the most commonly used form, is a stereoisomer of physiological thyroxine, which is metabolised more slowly and hence usually only needs once-daily administration.

Thyronamines have no medical usages yet, though their use has been proposed for controlled induction of hypothermia which causes the brain to enter a protective cycle, useful in preventing damage during ischemic shock.

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Quantum sufficit: just enough
From American Family Physician, 5/15/03 by Sarah Evans

* Eradicating world hunger is a slow struggle. According to New Scientist, the Food and Agriculture Organization (FAO) of the United Nations predicts that 440 million persons will be chronically hungry in the year 2030, down from 776 million now. This prediction revises the goal of the 1996 World Food Summit, which was to reduce the number of undernourished persons by one half by 2015; at the current rate, however, the number will fall only 11 percent by that year.

* Birth size, particularly length and head circumference, is associated with higher rates of breast cancer in premenopausal women. Study results published in BMJ showed that women who weighed more than 4,000 g (8 lb, 13 oz) at birth had a 3.5-fold higher rate of breast cancer than women of similar gestational age who weighed less than 3,000 g (6 lb, 10 oz) at birth. The lives of 5,358 women born between 1919 and 1925 were tracked from birth until the 1960 census. The study found no association between birth characteristics and breast cancer rates in postmenopausal women.

* The death of a child often shortens the life of the mother, according to a follow-up Danish study published in The Lancet. Researchers compared mortality rates over 16 years in 21,062 parents with a child who died before the age of 18 years and 293,745 parents who had not lost a child. Overall mortality was increased in the bereaved mothers, who were more likely to die of unnatural and natural causes than mothers who had not lost a child. Researchers found an increase in deaths from unnatural causes during the first three years of bereavement. The risk was particularly acute in the women whose children had died unexpectedly or from something other than illness. The study found that fathers also had an increased risk of death from unnatural causes in the first three years after losing a child.

* Patients who are prescribed liothyronine sodium for the treatment of depression should be monitored for signs of thyrotoxicosis, according to a case study published in Southern Medical Journal. During a routine gynecology appointment, a 49-year-old woman complained of fatigue, lethargy, weight gain, and occasional palpitations. Test results were consistent with central hypothyroidism, and she was referred to an endocrine clinic. A week before the clinic visit, the patient's triiodothyronine levels were found to be elevated. On evaluation at the clinic, it was discovered that she had been taking liothyronine sodium for seven months as an adjunct to fluoxetine prescribed by a psychiatrist.

* A beard--or the lack of one--may say a lot about a man's health. According to a study of 2,438 men (ages 45 to 59) that was published in the American Journal of Epidemiology, men who shave infrequently have a greater risk of all-cause and cardiovascular disease mortality and stroke events than men who shave every day. The researchers found that infrequent shavers are more likely to smoke than other men and are likely to share other characteristics that affect mortality and disease risk, such as having angina, being unmarried, and working in manual occupations. Smoking and social factors did not explain the increased risk of stroke in infrequent shavers, but hormone levels may be a factor.

* Keep your hands clean and your time free. As reported in AAP News, the U.S. Centers for Disease Control and Prevention (CDC) has suggested that physicians use 60 to 90 percent alcohol solutions to clean their hands if they are not visibly soiled. Compared with soap-and-water handwashing, using alcohol-based gels can save as much as an hour a day for physicians who normally follow CDC recommendations to wash soiled hands with soap and water for 15 seconds.

COPYRIGHT 2003 American Academy of Family Physicians
COPYRIGHT 2003 Gale Group

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