Thyroid gland and Parathyroid gland

 

The principle hormones secreted in thyroid gland are, thyroxine (T4), triiodothyronine (T3), reverse triiodothyronine (RT3), Calcitonin. T4 and T3 are synthesized in the colloid by iodination and condensation of tyrosine molecules bound in peptide linkage in thyroglobulin. Thyroglobulin is synthesized in the thyroid cells and secreted into the colloid by exocytosis of granules. The hormones remain bound to thyroglobulin until secreted. When they are secreted colloid is ingested by the thyroid cells, the peptide bounds are hydrolyzed and free T4 and T3 are discharge into the capillaries.  The lobes of the gland and as well as the isthmus contain many small globular sacs called, follicles. Follicles are lined with follicular cells and are filled with a fluid known as colloid that contain thyroglobulin. The follicular cells contain the enzyme needed for synthesize thyroglobulin, to thyroid hormone from thyroglobulin. When thyroid hormones are needed, thyroglobulin is reabsorbed from the colloid in the cells, where it is spit into its hormones. The hormones are then released, passing from the cells into the circulation.

Parafollicular cells (C- cells).

 They are in as singly or in groups. It is secreted among follicular cells, within basement membrane of follicles. It has unstained cytoplasm, pale granular cytoplasm. It secretes the hormone Calcitonin.

Thyroid cells have 3 functions,

-        Collect and transport iodine

-        Synthesized thyroglobulin and secrete it into the colloid

-        Remove the thyroid hormones from thyroglobulin and secrete them into the circulation.

Thyroid hormone synthesis.

Na ions / I ions symporter transports two Sodium ions across the basement membrane of the follicular cells along with an iodate ion. This is a secondary active transporter that utilizes the concentration gradient of Na ions to move Iodine ions against its concentration gradient. I ion is moved across the apical membrane into the colloid of the follicle by Pendrin. Thyroperoxidase oxidizes two I ions to form Iodine. Oxidized Iodine bounds to the thyrosine molecule attached to thyroglobulin. Monoiodotyrosine is next iodinated to form diiodothyrosine. Two diiodityrosine molecules form T. T3 is formed by condensation of monoiodotyrosine with diiodotyrosine. Iodine is nonreactive and only the more reactive iodine is required for the next step. The thyroperoxidase iodinates the tyrosyl residues of the thyroglobulin within the colloid. The thyroglobulin was synthesized in the endoplasmic reticulum of the follicular cell and secreted into the colloid by exocytosis. Iodinated thyroglobulin binds megalin for endocytosis back to the cell. Thyroid stimulating hormone (TSH) released from the anterior pituitary binds the TSH receptor on the basolateral membrane of the cell and stimulates the endocytosis of the colloid. The endocytosed vesicles fuse with the lysosomes of the follicular cell. The lysosomal enzymes cleave the T4 from the iodinated thyroglobulin. The thyroid hormones cross the follicular cell membrane towards the blood vessels by an unknown mechanism.

Large amounts of plasma T4 and T3 are bound to proteins. The free thyroid hormones in plasma are in equilibrium with the protein bound thyroid hormones in plasma and tissue. The free thyroid hormones in plasma are in equilibrium with the protein bound thyroid hormones in plasma and tissue.

-        Albumin: thyroxin- binding prealbumin (TBPA)

-        Thyroxin- binding globulin (TBG)

-        Free thyroxine index (FT4I)

-        Free triiodothyronine index