Editing Thyroid (section)

=== Thyroid hormone ===

** Synthesis
*** Occurs within the thyroid follicle unit, and is dependent on the presence of iodine
*** Follicular cells actively transport iodide anion across the cell membrane from the bloodstream into the cytoplasm via the Na/I symporter membrane protein, achieving a concentration within the follicular cells much higher than the systemic circulation
*** Iodide moves towards the follicular cell border with the colloid stores, and anions are oxidised to form the neutral I2 molecule, which can pass through the cell membrane into the colloid.
*** Colloid stores Tg, which contains a multitude of tyrosine residues. Colloid also contains the enzyme thyroid peroxidase (TPO), which catalyzes the next major step in thyroid hormone synthesis, which is the iodination of tyrosine residues on Tg, creating the molecule mono-iodotyrosine or di-iodotyrosine (DIT).
*** These two molecules form covalent bonds with one another to constitute the active forms of thyroid hormone tetra-iodothyronine or thyroxine (T4) (formed by two DITs and carrying four iodine molecules), or tri-iodothyronine (T3) (formed by a DIT and mono-iodotyrosine).
*** When stimulated by TSH, the follicular cells transport the activated thyroid hormones from the colloid centre into the bloodstream.
**
** Function
*** >99% of circulating thyroid hormones are bound to transport proteins such as albumin and T4-binding globulins; <1% exists in free form
*** T3 is the more potent form, but there is normally a higher concentration of T4 compared to T3
*** Many target organs can convert T4 to T3 through the deiodinase system
*** T4 and T3 bind to mitochondrial receptors leading to increased ATP production and energy consumption to produce heat - primary driver of basal metabolic rate
*** Critical for normal development, especially neurological
*** Increases cardiac output and vasodilation
*** Influences normal reproductive function
** Regulation
*** Low circulating T4 and T3 stimulate the hypothalamus to release thyrotropin-releasing hormone, which in turn stimulates the release of thyroid stimulating hormone from the anterior pituitary
*** TSH stimulates the formation of thyroid hormones by binding to its receptor on thyroid follicular cells - increased transport of iodine, as well as transport and release of T4 and T3 from the colloid into the bloodstream
*** Elevated T4 and T3 causes the reverse effect
** Iodine
*** Average daily requirement 0.1mg, all through diet
*** Thyroid uses almost all of it