Study note
SurgeryThe thyroid gland

Thyroid hormone synthesis, transport and activity

THE CONTEXT
Tri-iodothyronine (T3) and l-thyroxine (T4) are stored with thyroglobulin in colloid and released into blood when required.

Synthesis within thyroglobulin

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Thyroid hormone synthesis occurs within the thyroglobulin complex through distinct enzyme-controlled steps.
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Inorganic iodide is trapped from the blood and oxidised to iodine.
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Iodine binds to tyrosine to form iodotyrosine.
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Monoiodotyrosines and di-iodotyrosines couple to form T3 and T4.

Release and blood transport

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When hormones are required, the complex is resorbed into the cell and thyroglobulin is broken down, releasing T3 and T4 into blood.
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Circulating thyroid hormones bind to albumin, thyroxine-binding globulin (TBG) and thyroxine-binding prealbumin (TBPA).
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The small fraction remaining free in serum is biologically active and produces the metabolic effects.

Differences between T3 and T4

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Free T3 and free T4 constitute 0.3% and 0.03% of their respective total circulating hormones.
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T3 is the more important physiological hormone and is also produced in peripheral tissues by conversion from T4.
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T3 acts within a few hours, whereas T4 acts more slowly, over 4–14 days.
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