Located in the front of the neck in the human body, the thyroid gland is an endocrine gland responsible for the production of the thyroid hormones triiodothyronine (T3) and tetraiodothyronine, also known as thyroxine (T4). The thyroid hormone exerts its effects throughout the entire body; it plays an essential role in metabolism, bone growth, as well as many other processes and functions within the body. Most of the regulatory and metabolic actions of thyroid hormones are carried out by T3, which is the active form. T4 is mostly inactive and has to be converted in to active T3 before it can exert it effects in the body.
Unlike most other hormones, the effects of the thyroid hormone are widespread throughout the entire body and not limited to a single organ or body system. Some effects of thyroid hormone include:
- Cardiovascular system: Thyroid tablets causes and increase in the expression of beta-receptors in the body. This increased expression has positive chronotropic and inotropic effects of the cardiac muscles resulting in an increased cardiac output, stroke volume, and resting heart rate. There is also an increased inotropic effect as thyroid hormone causes an increase in intracellular calcium within the myocardium. Additionally, thyroid tablets cause a decrease in peripheral vascular resistance through the dilatation of the cardiac, skin, and muscle vessels. Furthermore, thyroid hormones activate the renin-angiotensin-aldosterone system which results in an increased blood volume due to an increase in fluid reabsorption.
- Respiratory system: T3 enhances the production of erythropoietin and hemoglobin which thereby increases the amount of oxygen delivered to the body tissues. It enhances the production of erythrocytes by improving folate and cobalamin absorption from the gastrointestinal tract; this allows for an increased capacity in oxygen delivery to tissues. The active thyroid hormone also stimulates resting respiratory rate as well as minute ventilation in instances of increased oxidation to ensure that blood oxygen levels remain within its normal range.
- Metabolism: T3 is directly responsible for the basal metabolic rate (BMR) and heat production (thermogenesis) in the body. Increased T3 levels results in an increased BMR as well as thermogenesis which is the pathogenesis behind the excessive sweating associated with hyperthyroidism. It also stimulates carbohydrate catabolism, protein anabolism, as well as lipid synthesis.
- Neurological system: Within the peripheral nervous system, T3 promotes an increase in peripheral reflexes and gastrointestinal motility; the increased reflexes is also due to the effect of T3 on increasing the production of Type II (fast twitch) muscle fibers. Additionally, T3 helps in increased alertness and responsiveness to the external environment. Furthermore, T3 is important in brain development because it stimulates the growth of axons within the neuron and myelin sheath formation.
- Skeletal system: Within the maternal uterus, T3 is key in the developing fetal growth centers. In children, T3 is needed for linear bone growth, endochondral ossification and maturation of the epiphyseal bone centers. T3 deficiency in childhood may result in stunting of bone growth and development; this may become permanent even after the administration of supplemental thyroid hormone.
- Reproductive system: T3 plays an essential role in the reproductive health in both men and women the regulation of the regulatory cycle and spermatogenesis respectively.
Given the wide-ranging roles of thyroid hormone in the human body, it is essential that its levels are well-maintained within the recommended range. Generally, normal levels of T3 should fall between 100 – 200 ng/dL, depending on the age and gender of the individual. When serum T3 levels fall below the recommended range, a condition known as hypothyroidism, oral thyroid supplementation may be warranted to prevent the disruption of systems and processes dependent on this hormone.