NTC Iodine in the Thyroid

Iodine / Iodide ingested - Some can convert Iodine to Iodide but many
have trouble doing this.

Iodide travels to the small intestine and is absorbed into the blood
stream.

Blood passes through the Thyroid where it traps the iodide into the
follicles via pendrin and NaI.

If there is a deficiency of iodine the thyroid enlarges in an attempt to
trap more iodine.

TSH stimulates a basolateral plasma membrane protein (NIS) for
accumulation of I- acting at the level of NIS gene transcription

NIS gene expression in the thyroid cells are regulated by thyrotropin
(TSH) via the cAMP dependent pathway.

mRNA expression is enhanced by TSH & cAMP and decreased by
iodide.

I- is carried across the basolateral membrane via ATP into the follicular
cells

The concentration is dependant on the electro chemical gradient of
Na+.
This gradient is maintained by the Na+/K+ ATPase

Once inside the cell iodide is oxidzed into iodine. This means that one
electron has been dropped. This process occurs with the reaction of
hydrogen peroxide (H2O2) with thyroperoxidase (TPO). Abnormalities
here can cause the body to create Anti-TPO antibodies. This leads to
Hashimoto's disease

Transported iodine is organified as Tg on the rough endoplasmic
reticulum by the action of thyroperoxidases (TPO), which is an enzyme
that when expressed liberates iodine for the covalent bonding to
tyrosine (an amino acid) residue on thyroglobulin forming
monoiodotyrosine (MIT) and diiodotyrosine (DIT) or Iodotyrosines.

DIT - Tyrosine with two iodine atoms added.
MIT - Tyrosine with one iodine atom added

The thyroglobulin iodination occurs in the colloid.

Coupling of the iodotyrosyl residues forms T4 (2-DIT) & T3 (1-DIT &
1-MIT)

Proteases digest the iodinated thyroglobulin releasing T4 & T3
hormones

Each thyroglobulin residue contains two MIT, four DIT, two T4 and one
third T3, meaning only one in three thyroglobulins contain a T3.

T4 is converted in the tissue by deiodinases

Deiodinases are unusual in that the enzyme contains selenium, in the
form of a rare amino acid selenocysteine

Deficiency in deiodinases can mimic iodine deficiency.