Physiology of thyroid gland

The follicular cells are responsible for the elaboration of two hormones

1.                  Thyroxine (tetra iodothyronine- T4)

2.                  Tri-iodothyronine T3,

The follicular cells elaborate these hormones by chemical activity in their apical region. Thyroglobulin is produced from the endoplasmic reticulum of the thyroid follicular cells and packaged in the Golgi apparatus. It is then passed through the apex of the follicular cell into the follicular space. In the follicle it combines with iodine to form a complex that becomes the colloid in the follicle. This colloid is reabsorbed into the follicular cells and then processed through activity of enzymes to form T3 (tri-iodothyronine) and T4 (thyroxine). T3 and T4 are they are released into the blood stream where they are bound to albumin in the blood and this time they can be assayed as plasma bound iodine (PBI).   Before they reach the tissues where they act, the albumin is again released from the hormones.   The hormones then freely enter into tissue cells where thyroxin is converted into active component known as tri-iodothyronine (T3).

Thyroxin and tri-iodothyronine are produced in concentration ratio of 3:1.

Follicular cells are cuboidal or squamous in shape when the cells are resting but tall and columnar when active.

The second type of cell is the parafollicular or C cell.   C cells are placed interfollicular, epifollicular or parafollicular in position and they are responsible for the elaboration of the hormone calcitonin which acts physiologically to reduce calcium levels in the blood stream.   This hormone also increases osteoblastic activity.

Cross section of thyroid gland
































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