They are chemical substances produced by the thyroid gland , which is found in the front of the neck.
This gland uses iodine to produce thyroid hormones, which are essential for the functioning of every cell in the body.
They help regulate the growth and speed of chemical reactions (metabolism) and participate in the circadian rhythms that govern sleep , among other essential functions.
The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3).
Thyroid stimulating hormone (TSH), which is produced by the pituitary gland, acts to stimulate the production of hormones by the thyroid gland.
The pituitary gland is stimulated to produce TSH by the gland of the hypothalamus in the brain.
The thyroid also produces the hormone calcitonin, which is involved in the metabolism of calcium and stimulates bone cells to add calcium to bone.
Thyroid hormone regulates the metabolic processes essential for normal growth and development, as well as regulating metabolism in adults.
It is well established that thyroid hormone status correlates with body weight and energy expenditure.
The hyperthyroidism , thyroid hormone excess, promotes a hypermetabolic state characterized by an increased energy expenditure at rest, weight loss, reduced cholesterol levels, increased lipolysis and gluconeogenesis.
In contrast, hypothyroidism , reduced levels of thyroid hormone, is associated with hypometabolism characterized by reduced energy expenditure at rest, weight gain, high cholesterol levels, reduced lipolysis and reduced gluconeogenesis.
Stimulates lipogenesis and lipolysis, although when the levels of this hormone are high, the net effect is the loss of fat.
It influences the key metabolic pathways that control the energy balance by regulating storage and energy expenditure.
It regulates metabolism mainly through actions in the brain, white fat, brown fat, skeletal muscle, liver and pancreas.
It is likely that all the cells in the body are targets for thyroid hormones.
Although not strictly necessary for life, thyroid hormones have profound effects on many physiological processes “in a big way”, such as development, growth and metabolism, and the deficiency of thyroid hormones is not compatible with normal health.
In addition, many of the effects of thyroid hormone have been delineated by the study of deficiency and excess of states.
Thyroid hormones stimulate various metabolic activities in most tissues, leading to an increase in the basal metabolic rate.
One consequence of this activity is to increase the production of body heat, which seems to result, at least in part, from the increase in oxygen consumption and the rates of ATP hydrolysis.
By way of analogy, the action of thyroid hormones is similar to blowing in a blazing fire.
Some examples of specific metabolic effects of thyroid hormones include:
The increase in thyroid hormone levels stimulates the mobilization of fat, which leads to an increase in the concentrations of fatty acids in the plasma.
They also improve the oxidation of fatty acids in many tissues.
Finally, plasma concentrations of cholesterol and triglycerides correlate inversely with thyroid hormone levels: a diagnostic indication of hypothyroidism is an increase in the concentration of cholesterol in the blood.
Metabolism of carbohydrates
Thyroid hormones stimulate almost all aspects of carbohydrate metabolism, including the improvement of insulin-dependent glucose entry into cells and the increase of gluconeogenesis and glycogenolysis to generate free glucose.
Thyroid hormones are clearly necessary for normal growth in children and young animals, as evidenced by the growth retardation observed in thyroid deficiency.
It is not surprising that the growth promoting effect of thyroid hormones is intimately intertwined with that of growth hormone, a clear indication that complex physiological processes such as growth depend on multiple endocrine controls.
A classic experiment in endocrinology was the demonstration that deprived thyroid hormone tadpoles could not undergo metamorphosis in frogs.
Of critical importance in mammals is the fact that normal levels of thyroid hormone are essential for the development of the fetal and neonatal brain.
As mentioned earlier, there do not seem to be organs and tissues that are not affected by thyroid hormones.
Some well-documented additional effects of thyroid hormones include:
- Cardiovascular system: thyroid hormones increase heart rate, cardiac contractility and cardiac output. They also promote vasodilation, which leads to improved blood flow to many organs.
- Central nervous system: both the decrease and increase in the concentrations of thyroid hormones cause alterations in the mental state. Very little thyroid hormone, and the individual tends to feel mentally slow, while too much induces anxiety and nervousness.
- Reproductive system: Normal reproductive behavior and physiology depend on essentially normal levels of thyroid hormone. Hypothyroidism in particular is commonly associated with infertility.
The disease is associated with inadequate production and overproduction of thyroid hormones. Both types of disease are relatively common afflictions of man and animals.
Hypothyroidism is the result of any condition that causes thyroid hormone deficiency.
Two well-known examples include:
- Iodine deficiency: iodide is absolutely necessary for the production of thyroid hormones; Without adequate iodine intake, thyroid hormones can not be synthesized.
- Primary thyroid disease : inflammatory diseases of the thyroid that destroy parts of the gland are clearly an important cause of hypothyroidism.
The common symptoms of hypothyroidism that arise after early childhood include lethargy, fatigue, intolerance to cold, weakness, hair loss and reproductive failure.
If these signs are severe, the clinical condition is called myxedema.
In the case of iodide deficiency, the thyroid becomes extraordinarily large and is called goiter .
The most severe and devastating form of hypothyroidism is seen in young children with congenital thyroid deficiency.
If that condition is not corrected by complementary therapy soon after birth, the child will suffer from cretinism , a form of irreversible growth and mental retardation.
Most cases of hypothyroidism are easily treated by oral administration of synthetic thyroid hormone.
In times past, the intake of dried thyroid glands from dried animals was used for the same purpose.
Hyperthyroidism results from the secretion of thyroid hormones.
In most species, this condition is less common than hypothyroidism.
In humans, the most common form of hyperthyroidism is Graves’ disease , an immune disease in which the autoantibodies bind and activate the thyroid stimulating hormone receptor, leading to the continued stimulation of the synthesis of the thyroid. thyroid hormone
Another interesting but rare cause of hyperthyroidism is called thyrotoxicosis .
The common signs of hyperthyroidism are basically the opposite of those seen in hypothyroidism, and include nervousness, insomnia , high heart rate, eye disease and anxiety.
Graves disease is commonly treated with antithyroid drugs (eg, propylthiourea, methimazole), which suppress the synthesis of thyroid hormones primarily by interfering with the iodination of thyroglobulin by thyroid peroxidase.