T3 and T4

(noun)

The thyroid hormones, triiodothyronine (T3) and thyroxine (T4), are tyrosine-based hormones produced by the thyroid gland that are primarily responsible for regulation of metabolism.

Related Terms

Examples of T3 and T4 in the following topics:

  • Action of Thyroid Hormones

    • The primary function of the thyroid is to produce the hormones triiodothyronine (T3), thyroxine (T4), and calcitonin.
    • Triiodothyronine (T3) and thyroxine (T4) are enzymes produced by the thyroid gland.
    • The cells of the developing brain are a major target for T3 and T4.
    • In the blood, T4 and T3 are partially bound to thyroxine-binding globulin (TBG), transthyretin, and albumin.
    • Only a very small fraction of the circulating hormone is free—T4 0.03% and T3 0.3%.

  • Formation, Storage, and Release of Thyroid Hormones

    • Thyroid hormones (T4 and T3) are produced by the follicular cells of the thyroid gland and regulated by thyroid-stimulating hormone (TSH).
    • Thyroglobulin, the pre-cursor of T4 and T3, is produced by the thyroid follicular cells before being secreted and stored in the follicular lumen.
    • Proteases digest iodinated thyroglobulin, releasing the hormones T4 and T3, the biologically-active agents central to metabolic regulation.
    • T3 is identical to T4, but it has one less iodine atom per molecule.
    • T4 is believed to be a pro-hormone and a reservoir for the more active and main thyroid hormone T3.

  • Overview of the Thyroid Gland

    • It participates in these processes by producing thyroid hormones, the principal ones being triiodothyronine (T3) and thyroxine (sometimes referred to as tetraiodothyronine (T4)).
    • These hormones regulate the rate of metabolism and affect the growth and rate of function of many other systems in the body.
    • T3 and T4 are synthesized from both iodine and tyrosine.
    • The thyroid gland is a butterfly-shaped organ and is composed of two cone-like lobes or wings, lobus dexter (right lobe) and lobus sinister (left lobe), connected via the isthmus.
    • The organ is situated on the anterior side of the neck, lying against and around the larynx and trachea, reaching posteriorly the oesophagus and carotid sheath.

  • Thyroid Gland Disorders

    • Hyperthyroidism, or overactive thyroid, is the overproduction of the thyroid hormones T3 and T4 .
    • These medications involve frequent dosing and close treatment monitoring.
    • Hypothyroidism is the underproduction of the thyroid hormones T3 and T4.
    • But when the hypothyroidism is caused by iodine insufficiency, the thyroid is unable to produce T3 and T4.
    • These occur at any age and might not become evident until adult life.

  • Control of Thyroid Hormone Release

    • The production of thyroxine and triiodothyronine is regulated by thyroid-stimulating hormone (TSH) that is released from the anterior pituitary.
    • The production of thyroxine (T4) and triiodothyronine (T3) is primarily regulated by thyroid-stimulating hormone (TSH) that is released from the anterior pituitary gland.
    • Thyroid hormones also provide negative feedback to the hypothalamus and anterior pituitary gland.
    • When thyroid levels in the blood are elevated TSH and TRH production is reduced.
    • Thyroid hormones provide negative feedback, inhibiting secretion of TRH and TSH when blood levels are high.

  • Overview of Endocrine Glands

    • A collection of endocrine glands makes up the endocrine system: the pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas and gonads.
    • Two hormones—oxytocin and antidiuretic hormone (ADH)—are produced and released this way.
    • Communication between the hypothalamus and the anterior pituitary occurs through hormones (releasing hormones and inhibiting hormones) that are produced by the hypothalamus and delivered to the anterior pituitary via a portal network of capillaries.
    • It is involved in the production of the hormones T3 (triiodothyronine) and T4 (thyroxine).
    • PTH is released directly into the bloodstream and travels to its target cells, which are often quite far away and found in bone, kidneys, and the gastrointestinal system.

  • Development of the Endocrine System

    • The endocrine system provides an electrochemical connection from the hypothalamus of the brain to the organs that control the body's metabolism, growth and development, and reproduction.
    • The main endocrine glands are the hypothalamus (neuroendocrine gland), pituitary (anterior and posterior lobes), thyroid, parathyroid, adrenal (cortex and medulla), pancreas, and gonads.
    • It is involved in the production of the hormones T3 (triiodothyronine) and T4 (thyroxine).
    • Thyroxine stimulates oxidative metabolism in cells and increases the oxygen consumption and heat production of most body tissues, with the notable exception of the brain.
    • This condition produces symptoms such as an abnormally high body temperature, profuse sweating, high blood pressure, weight loss, irritability, and muscular pain and weakness.

  • Pituitary Gland Disorders

    • Once the effector glands produce sufficient hormones (thyroxine, cortisol, estradiol or testosterone and IGF-1), both the hypothalamus and the pituitary cells sense their abundance and reduce their secretion of stimulating hormones.
    • The signs and symptoms of hypopituitarism vary, depending on which hormones are undersecreted and on the underlying cause of the abnormality.
    • If the onset is abrupt, collapse, shock and vomiting may occur.
    • Thyroid-stimulating hormone (TSH) deficiency leads to hypothyroidism (lack of production of thyroxine (T4) and triiodothyronine (T3) in the thyroid).
    • Typical symptoms are tiredness, intolerance to cold, constipation, weight gain, hair loss and slowed thinking, as well as a slowed heart rate and low blood pressure.