PHIS 206 Lecture Notes - Lecture 31: Thyroid Cartilage, Follicular Cell, Thyroglobulin
2 May 2018
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Lecture 31: Thyroid Gland & Ca2+ Metabolism
Thyroid Gland
• Thyroid Gland
o Butterfly-shaped, with isthmus connecting the lobes
o Sits inferior to thyroid cartilage, roughly where bow tie would rest
Thyroid Gland Histology
• Architecture
o Follicular Cell secrete thyroid hormone
o C cell (parafollicular cells) secrete calcitonin
o Colloid extracellular space bounded by follicular cells
Thyroid Hormone Synthesis
• Thyroglobulin
o Within the colloid, the protein thyroglobulin exists in high concentration
o Most of the synthesis of thyroid hormone takes place on the thyroglobulin
molecule
o Thyroglobulin rich in amino acid tyrosine
• Iodine
o Colloid doesn’t face the bloodstream, but the follicular cells do.
o Follicular cells extract iodide (I−) from the blood and convert it to elemental
iodine (I2)
o The iodine enters the colloid where the iodine is attached to tyrosines on
thyroglobulin.
• Thyroid Hormone
o Small pieces of thyroglobulin break free, with one iodine attached
(monoiodotyrosine - MIT) or two iodines attached (diiodotyrosine - DIT).
o DIT can then combine with MIT, forming triiodothyronine (T3)
o or DIT can combine with DIT, forming thyroxine (T4)
Thyroid Hormone (TH)
• Secretion
o The synthesized T3 and T4 is taken from the colloid by the follicular cells and
released into the blood stream
• Transport
o ↑ery little thyroid hormone (T3 and T4) circulates freely most is bound to
carrier proteins such as thyroxine-binding globulin (TBG)
• Differences between T3 and T4
o Both perform the same functions, both ”thyroid hormone” (TH) Far more T4 is
made (90% T4/10% T3), but T3 is 4X as powerful as T4
• About 10x as much T4 as T3, T3 is more powerful than T4 but T3 is stronger than T4
• T4 has 4 iodines and T3 has 3 iodines
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