IODINE
Iodine – is essential for thyroid function
The thyroid hormones triiodothyronine (T3) and thyroxine or tetraiodothyronine (T4) contain 3 and 4 iodine atoms respectively. T4 is about 50x more abundant in the blood than T3.
Inside the cell, T3 (the active form – produced locally from T4 by deiodinase) binds to a nuclear receptor and initiates a cascade of changes in gene transcription/mRNA stability.
The thyroid hormones are crucially important in growth and development, and regulate metabolic activity in all cells in the body.
T3 and T4 are crucially important for the development of most organs, and especially the brain.
The brain develops from the fetal stage until 3 years. Lack of iodine in this critical period leads to irreversible changes in the brain. Mental retardation is the main consequence.
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Nuclear receptors regulate gene expression.
Thyroid hormone receptors are member of the nuclear receptor superfamily described in e-lectures about vitamin A and vitamin D.
ANIMATION
To see again the animation of transcriptional activation mechanism of nuclear receptors shown in the previous e-lecture on Vitamin A, click here:
START ANIMATION
The thyroid gland
The thyroid normally weighs 15-25 g (in a healthy adult) and contains 70-80% of all the iodine in the body (15-20 mg).
Secretion of T3 and T4 is under the control of thyroid stimulating hormone (TSH) which is produced in the pituitary. In turn, TSH production is regulated by the serum level of T4. If T4 falls, TSH activity increases (a feedback mechanism).
If iodine is lacking, T4 production is reduced and TSH stays high, causing hyperplasia of the cells of the thyroid follicles, and enlargement of the gland (goitre).
In the enlarged gland, iodine content may be as low as 1 mg.
Goitre (Norwegian struma) is a non-specific term describing enlargement of the thyroid gland. It does not imply the presence of any specific pathology. Goitres can be either diffuse or multi-nodular.