PSL201 – Feb 26 – Endocrine 3 Thyroid Hormones.docx

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11 Apr 2012
PSL201 Feb 26 Endocrine 3: Thyroid Hormones
Thyroid Gland and Parathyroid Glands
- thyroid gland: butterflyshaped structure on the ventral side of the trachea
o secretes T3,/T4 (regulate metabolic rates), and calcitonin (regulates
calcium levels in blood)
- parathyroid glands: smaller structures on posterior surface of the thyroid
o secrete parathyroid hormone (PTH)
regulator of calcium levels in blood
- close to the heart and secretes thymosin
- critical for normal immune function
Thyroid Hormones
- plasma levels are relatively steady
- little change in levels
- work to “maintain” rather than “trigger”
Synthesis and Secretion of Thyroid Hormones
- hormones produced from follicles
o single outer layer of follicular cells surrounding a central protein rich
colloid secreted by the follicular cells
- C cells are located in the space between follicles
o Synthesize and secrete calcitonin
- Throboglobulin (TG): primary substance found in colloid that functions as a
precursor for thyroid hormones
Steps of thyrois hormone secretion and synthesis:
1) TG residues are iodinated forming monoiodotyrosine (MIT)
- Addition of a second iodide forms diiodotyrosine (DIT)
2) Two MIT or DIT on the same TG molecule are coupled and two tyrosine are
linked together by a double bond
- If 2 DIT cobine T4 is produced
- 1 MIT and 1 DIT combine, T3 is produced
- 2 MIT cannot combine
3) throid hormones are stored in the colloid bound to TG for up to 3 months
4) TSH arriving via bloodstream binds to receptors on follicular cells activating the
cAMP system. Results in the phosphorylation ofcell proteins necessary for T3/T4
5) follicular cells take iniodinated TG molecules from the colloid by phagocytosis
6) phagosome containing ionodated TG fuses with a lysosome
7) T3 and T4 is released into the follicular cell
- diffuse across the membrane (hydrophobic) and into bloodstream carried
by thyroxin binding globulin and transthyretin
- T4 is produced 4x more than T3
o Yet T3 is 4x mor potent
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