As a result of this inhibition, serum thyroid hormone concentrations are able to fall toward normal levels. The complex interactions between thyroid hormone and thyrotropin maintain serum thyroid hormone concentrations within narrow limits.
This explains the tachycardia associated with high levels of T3 and T4.When the levels of T3 and T4 are high, the increase in BMR will result in an increase in temperature.
During fetal life and in infancy this stimulatory activity of triiodothyronine is critically important for normal neural and skeletal growth and development; in both the unborn and the newborn, thyroid deficiency is associated with dwarfism and intellectual disability.
The iodide trap is a pump in the follicles that actively pumps iodide from the blood into the follicles. This is shown in the diagram below. Once the follicular cells have taken up the iodide, it is activated into a reactive form by a peroxidase enzyme.
Since thyroid hormones are essential for physical and mental development, hypothyroidism during development (ie, before birth and during childhood) can result in learning difficulties and reduced physical growth. Hypothyroidism in adults results in decreased metabolic rate. This
Oestrogens increase the synthesis of TBG and decrease the clearance of the iodothyronines. In states where there are high levels of circulating oestrogens (i.e. pregnancy there are high levels of circulating iodothyronines.
Carbohydrate Metabolism. T3 and T4 increase all aspects of carbohydrate metabolism. Protein Metabolism Iodothyronines stimulate both protein synthesis and degradation. High levels of T3 and T4 will result in more protein degradation compared to protein synthesis.
Of thyrotoxicosis include intolerance to heat, weight loss, increased appetite, increased bowel movements, irregular menstrual cycle, rapid and irregular heartbeat, palpitations, tiredness, irritability, tremor, hair loss and retraction of the eyelids resulting in a staring appearance.
It also stimulates carbohydrate utilization, lipid production and metabolism (thereby increasing cholesterol utilization and central and autonomic nervous system activation, resulting in increased contraction of cardiac muscle and increased heart rate.
Hypothyroidism is often accompanied by an enlargement of the thyroid gland known as goitre. Thyrotoxicosis is the term given when there is too much thyroid hormone in the bloodstream. It may be a result of overactivity of the thyroid gland (hyperthyroidism) as in Graves disease, inflammation of the thyroid or a benign thyroid tumour. Symptoms
There is another important conversion in the periphery where T4 is deiodinated to an inactive iodothyronine called reverse T3 (rT3). The precise role of rT3 is not fully understood, but it is thought to regulate the amount of active iodothyronines in the periphery.
These two hormones are the only biologically active substances that contain iodine, and they cannot be produced in the absence of iodine. The process leading to the eventual synthesis of thyroxine and triiodothyronine begins in the thyroid follicular cells, which concentrate iodine from the serum.
Regulation of thyroid hormone secretion The thyroid gland is one component of the hypothalamic-pituitary-thyroid axis, which is a prime example of a negative feedback control system. The production and secretion of thyroxine and triiodothyronine by the thyroid gland are stimulated by the hypothalamic hormone thyrotropin-releasing hormone and the anterior pituitary hormone thyrotropin.
This hormone production system is regulated by a negative feedback loop so that when the levels of the thyroid hormones, thyroxine and triiodothyronine increase, they prevent the release of both thyrotropin -releasing hormone and thyroid stimulating hormone. This
Causes symptoms which include fatigue, intolerance of cold temperatures, low heart rate, weight gain, reduced appetite, poor memory, depression, stiffness of muscles and infertility. Reviewed: January 2015.