1. What is the basic histologic structure of the thyroid gland?

• The thyroid is composed of numerous follicles that have a central fluid-filled cavity. They are lined with follicular cells that secrete the main hormones

• Interspersed among the follicles are the parafollicular cells

2. Which hormones does the thyroid produce?

• Tetra-iodothyronine (T4, thyroxine): the principle hormone of the thyroid gland

• Tri-iodothyronine (T3): measure for measure, this is more potent than T4, however, has a shorter duration of action

• Calcitonin: produced by the para-follicular cells. This is important in the regulation of serum calcium (see ‘Calcium balance’)

3. Name another source of T3 other than the thyroid.

This may also be produced by the conversion of T4 in the peripheral tissues. In fact, the thyroid accounts for only 20% of the extrathyroid pool of T3.

4. Which other hormone may be produced following the peripheral conversion of T4?

Reversed-T3 (r-T3). This is an inactive hormone acts as a point of peripheral thyroid hormone control.

5. Outline the steps involved in the production of T3 and T4.

• Iodide trapping: dietary iodine is concentrated into the follicular cells by an active pump mechanism

• Oxidation: of iodide to a reactive form by the enzyme peroxidase. This is located on the apical membrane

• Organification: through binding with amino acids – mainly tyrosine. These form tyrosyl units

• Thyroglobulin formation: tyrosyl units combine with a protein core to form thyroglobulin

• Internal coupling: tyrosyl units combine on the thyroglobulin molecule to form T3 or T4 molecules still bound to the protein core

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