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 production of the hormones triiodothyronine (T3), thyroxine (T4), and calcitonin.
    • Triiodothyronine (T3) and thyroxine (T4) are enzymes produced by the thyroid gland.
    • 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%.

  • Humoral, Hormonal, and Neural Stimuli

    • The anterior pituitary releases the thyroid-stimulating hormone (TSH), which then stimulates the thyroid gland to produce the hormones T3 and T4 .
    • As blood concentrations of T3 and T4 rise, they inhibit both the pituitary and the hypothalamus in a negative feedback loop.
    • Recall that in a short-term stress response, the hormones epinephrine and norepinephrine are important for providing the bursts of energy required for the body to respond.
    • Here, neuronal signaling from the sympathetic nervous system directly stimulates the adrenal medulla to release the hormones epinephrine and norepinephrine in response to stress.
    • This in turn causes the release of the hormones T3 and T4.

  • Thyroid Gland

    • The thyroid gland, the largest endocrine gland, is responsible for the production of the hormones T3, T4, and calcitonin.
    • The thyroid gland produces the hormones T3 (triiodothyronine) and T4 (thyroxine).
    • Follicle cells are stimulated to release stored T3 and T4 by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary.
    • The location of the thyroid gland is in the neck below the larynx and in front of the trachea; it is the largest endocrine gland in the body, producing T3, T4, and calcitonin.
    • Describe the hormones produced by the thyroid and explain how their production is regulated

  • 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 TSH (thyroid stimulating hormone).
    • 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.

  • Hormonal Regulation of Metabolism

    • The levels of glucose in the blood are regulated by the hormones insulin and glucagon from the pancreas, and T3 and T4 from the thyroid.
    • Over time, high blood glucose levels can cause nerve damage to the eyes and peripheral body tissues, as well as damage to the kidneys and cardiovascular system.
    • The basal metabolic rate, which is the amount of calories required by the body at rest, is determined by two hormones produced by the thyroid gland: thyroxine, also known as tetraiodothyronine or T4, and triiodothyronine, also known as T3.
    • T3 and T4 release from the thyroid gland are stimulated by thyroid-stimulating hormone (TSH), which is produced by the anterior pituitary.
    • This results in increased rates of metabolism and body heat production.

  • Overview of the Thyroid Gland

    • It participates in these processes by producing thyroid hormones, the principal ones being triiodothyronine (T3) and thyroxine which is 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.

  • Alkyl Halide Occurrence

    • The thyroid hormones T3 and T4 are exceptions; as is fluoroacetate, the toxic agent in the South African shrub Dichapetalum cymosum, known as "gifblaar".
    • Synthetic organic halogen compounds are readily available by direct halogenation of hydrocarbons and by addition reactions to alkenes and alkynes.
    • Because DDT is a cheap and effective mosquito control agent, underdeveloped countries in Africa and Latin America have experienced a dramatic increase in malaria deaths following its removal, and arguments are made for returning it to limited use. 2,4,5-T and 2,4-D are common herbicides that are sold by most garden stores.
    • Other organic halogen compounds that have been implicated in environmental damage include the polychloro- and polybromo-biphenyls (PCBs and PBBs), used as heat transfer fluids and fire retardants; and freons (e.g.
    • CCl2F2 and other chlorofluorocarbons) used as refrigeration gases and fire extinguishing agents.

  • 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), released by the anterior pituitary.
    • The production of thyroxine (T4) and triiodothyronine (T3) is primarily regulated by thyroid-stimulating hormone (TSH), released by 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.

  • Virulent Bacteriophages and T4

    • Model organisms of virulent viruses that have been extensively studied include virus T4 and other T-even bacteriophages which infect Escherichia coli and a number of related Bacteria.
    • T4 is a relatively large phage, at approximately 90 nm wide and 200 nm long (most phages range from 25 to 200 nm in length).
    • Its tail fibres allow attachment to a host cell, and the T4's tail is hollow so that it can pass its nucleic acid to the cell it is infecting during attachment.
    • T4 is capable of undergoing only a lytic lifecycle and not the lysogenic lifecycle.
    • Complementation, deletion, and recombination tests can be used to map out the rII gene locus by using T4.