Sun28November0333PM 7

What is the principal site of storage of the thyroid hormones in the thyroid gland?

(Please select 1 option)

Within vesicles

Bound to colloid

Thyroglobulin Correct

Complexed in the follicular cell

In free form

Explanation

The functional unit of the thyroid gland is the follicle. These follicular cells surround the follicle and the latter is filled with a fluid called colloid. Suspended within the colloid are pro-hormone complexes, thyroglobulin.

The role of synthesis and storage of thyroid hormones is undertaken by the follicular cells and the thyroglobulin within the colloid.

Under the action of thyroid stimulating hormone (TSH), iodide ions (I-) are actively transported against a concentration gradient into the follicular cell. It is here that it undergoes oxidation to "active" iodine by thyroid peroxidase (TPO). Thyroglobulin itself is synthesised in the follicular cells and contains up to 140 tyrosine residues. Active iodine is incorporated into the tyrosine residues of thyroglobulin to form mono- and di-iodotyrosines (MIT and DIT). The iodinated thyroglobulin is then taken up into the colloid where it is stored and dimerised. The coupling of two DIT molecules produces thyroxine (T4) and the coupling of one MIT and one DIT molecule produces tri-iodotyrosine (T3). This process is catalysed by TPO.

Stimulated by TSH, thyroglobulin droplets are captured by the follicular cells by pinocytosis to form vesicles. Fusion of these vesicles with lysosomes results in hydrolysis of the thyroglobulin molecules and release of T4 and T3 into the circulation.

Answer Statistics

1

24%

2

15%

3

46%

4

17%

Times answered: 265