THYROID
ANATOMY
The
thyroid is developed from the thyroglassal duct which starts at the base if the
tongue (foramen cerium) the rest of the duct disappear leaving the two lobes
the isthmus and pyramidal lobby.
The
bilobed gland weighs 20-25gms. The gland
is placed on either side of the larynx on the thyroid cartilage and the upper
rings of the trachea a lobule has 20-40 follides lined with epthelium. The
enclosed space contains thyroglobalin. The gland has an extensive network of lymphatics
arteries and veins.
1. METABOLISM OF IODIDE
- Formation
of normal quantities of thyroid hormone ultimately depends upon the
availability of adequate quantities of exogenous iodine.
- Normally
iodine balance is maintained through dietary sources.
- Daily
dietary intake of iodine depends on the iodine content of soil and water
through which iodine enters our food chain i.e. vegetables, animals and
drinking water.
- Iodine
is ingested in both the inorganic and the organically bound forms.
- Eventually
it is made available as inorganic iodide
- Iodide
itself is rapidly and efficiently absorbed from the gastrointestinal tract, and
little is lost in the stool.
- Iodine
used in the synthesis of thyroid hormone is drawn from the inorganic iodide of
the extracellular fluid.
- The
iodide that is cleared in the urine is partly replenished both by iodide lost
from the thyroid into the blood from unconjugation of tyrosine and by iodide
liberated through deiodination of thyroid hormones in peripheral tissues.
Normal
iodide intake in the west is about 500 µg of iodine is cleared into the urine
daily, in the inorganic form. The second major site of removal of iodine from
the extracellular fluid is the thyroid. Iodide removed from plasma by the thyroid
is not lost, since ultimately it is secreted into the circulation either as
iodinated thyronines T4 and T3.
Under
normal circumstances, approximately 8000µg of iodide is in the form of
iodinated amino acids.
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