Thyroid gland: Histology and Hormones

  • An endocrine gland situated at the root of the neck, one on either side of the trachea.
  • Bi-lobed, connected to each other by an isthmus.
  • Weight is about 20-40gm, 4 to 4.8 x 1 to 1.8 x 0.8 to 1.6 cm in size, in adults.
  • Larger in females as compared to males.
  • Changes occur in structure and function in different stages of sexual cycles in females.
  • Pregnancy and lactation increases its function slightly.
  • In menopause, its function declines.

Thyroid Gland- Definition, Structure, Hormones, Functions, Disorders

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Histology of Thyroid gland

  • Contains large number of closed follicles lined with cuboidal epithelial cells.
  • So the cells are called follicular cells.
  • Thyroglobulin, a colloidal substance, is secreted by these follicular cells.
  • This substance fills the follicular cavity.
  • Tetra-iodothyronine (T4 or thyroxine) and tri-iodothyronine (T3) are also secreted by these cells.
  • Parafollicular cells present in between follicles which secretes calcitonin.

Comprehensive Review of Thyroid Embryology, Anatomy, Histology, and  Physiology for Surgeons

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Hormones of Thyroid gland

  • T4, T3 and calcitonin are the main hormones secreted.
  • T4 also called thyroxine forms about 90% of secretion.
  • T3 forms only 9-10% of total secretion.
  • T4 and T3 are derivatives of amino-acids tyrosine which contains iodine.
  • T3 has four times more potency than T4 though T3 acts for shorter period than T4.
  • T4 has more affinity and binds strongly with plasma proteins so released slowly and acts longer.
  • T3 has less affinity with plasma proteins and binds loosely, so released quickly and acts shorter.
  • Therefore, T4 acts slowly on the target cells than T3.
  • T4 has half-life of 7 days which is longer than in T3 i.e. T3 has between 10 and 24 hours.
  • T4 is secreted more i.e., 80-90 μgm/day whereas T3 is 4-5 μgm/day.
  • Reverse T3 is secreted only about 1-2 μgm/day.
  • Plasma level of T4 is more i.e., 8 μgm/dl whereas T3 is 0.12 μgm/dl.
  • In muscles, liver and kidney, thyroid hormones generally degrades.

Thyroid Hormones Synthesis

  • In thyroglobulin present in follicular cavity, thyroid hormones synthesis occurs.
  • Iodine and tyrosine from diet absorbed from GI tract, are essential materials for its formation.
  • Iodine is converted to iodide.
  • 1mg of iodine per week or about 50 mg/year iodine is required.
  • Salt is iodized with one part of sodium iodide to every 100,000 parts of sodium chloride to prevent iodine deficiency.
  • In five stages, synthesis of these hormones occurs. They are:

Thyroid - Part 3 - Thyroxine Binding Globulin (TBG), Thyroid Binding  Globulin -

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a) Thyroglobulin synthesis

  • Thyroglobulin is secreted by ER and golgi apparatus of the follicular cells continuously.
  • Each thyroglobulin molecule contains 140 molecules of amino acid tyrosine.
  • Thyroglobulin is stored in the follicle after synthesis which is a large glycoprotein.

b) Iodide trapping

  • From blood, iodide is carried to follicular cell against electrochemical gradient.
  • This process is known as iodide trapping.
  • Sodium-iodine symport pump helps to transport iodide along with sodium.
  • This pump is also called iodide pump.
  • Iodide is 30 times more concentrated in thyroid than in blood.
  • Hyperactivity of thyroid gland increases iodide concentration by more than 200 times.

c) Iodide oxidation

  • Iodine is required to combine with tyrosine to form thyroid hormones.
  • So iodide is oxidized to form iodine which is the elemental form.
  • This oxidation process occurs in follicular cells.
  • It occurs in presence of thyroid peroxidase.
  • This enzyme absence or inactivity stops thyroid hormones formation.

d) Transport of iodine to follicular cavity

  • Iodine is transported to follicular cavity later on by an iodide-chloride pump.
  • This pump is also called pendrin.

e) Iodination of Tyrosine

  • Iodination means combination of iodine with tyrosine.
  • It occurs in thyroglobulin.
  • Iodine is first carried to follicular cells into the follicular cavity.
  • In this cavity, iodine binds with thyroglobulin.
  • This process is called organification of thyroglobulin.
  • Tyrosine present in the thyroglobulin then combines with iodine.
  • Iodinase enzyme secreted by follicular cells accelerates this iodination process.
  • This process occurs in different stages.
  • Firstly, monoiodotyrosine (MIT) is formed which later converted to diiodotyrosine.
  • MIT and DIT are also known as iodotyrosine residues.

f) Coupling reactions

  • The above residues get coupled with each other.
  • Coupling occurs in different configurations to give different thyroid hormones.
  • Coupling reactions includes following steps:

i) One molecule MIT and one molecule of DIT forms tri-iodothyronine(T3).

ii) Sometime the above reaction yields another form of T3 called reverse T3 or rT3.

iii) This form of T3 is 1% of thyroid output.

iv) Two molecules of DIT form tetra-iodothyronine which is also called thyroxine.

Storage of thyroid Hormones

  • Remains in the form of vesicles within thyroglobulin.
  • Stored here in this form for longer period.
  • 5-6 molecules of thyroxine are present in each thyroglobulin molecule.
  • In average, 1 triiodothyronine molecule is present for every 10 molecules of thyroxine.
  • In combination form, the thyroid hormones can be stored for long time i.e., for several months.
  • This is the only endocrine gland for storing hormones for about 4 months.
  • Signs and symptoms of deficiency to these hormones, do not appear for about 4 months though synthesis of hormones stops.






Thyroid gland: Histology and Hormones