Thyroid Histology

Thyroid Histology
Dr A McLeod

The thyroid is one of the most recognisable tissues of the body - this means that it is ideal for simple 'what is this tissue' questions.

The top picture shows the thyroid gland at a low magnification. You can see that homogenous 'islands' are surrounded by walls of nucleated cells. The white, often crescent shaped areas within the follicles are artifacts of slide preparation.

The middle picture is at a higher magnification - you can clearly see the nucleated cells that form the walls of the follicles.

In the bottom picture these structures are labelled.

The follicular cells

Manufacture the glycoprotein component of thyroglobulin and secrete it into the follicle.
Change iodide into iodine and secrete it into the follicle where it combines with the glycoprotein spontaneously to form thyroglobulin.
At need, break down the long thyroglobulin molecule into smaller T3 and T4 and releases it into the blodsream.

C-cells

These cells lie between follicles within the thyroid gland. They are inobvious in humans with a slightly paler staining nucleus. Their purpose is production and release of calcitonin.
Calcitonin has the opposite effect to parathormone - it reduces the action of osteoclasts and increases activity of osteoblasts.
It is produced in response to calcium levels alone. It is not adirectly ffected by phosphate levels or other hormones.

Functions of thyroid hormone

Both T3 and T4:

Increase cell metabolism
Facilitate normal growth
Facilitate normal mental development
Increase the local effects of catecholamines
Are present in the bloodstream bound to thyroid binding globulin (TBG), thyroxine binding pre-albumin (TBPA) and albumin. The unbound form is the active one and the molecule gains entry to cells by an ATP dependent method.

T3 is the most active form of the hormone

Its half-life is about 24 hours
About 5x more active than T4
About 85% of circulating T3 from monodeiodination of T4 in the tissues of the body especially the liver, muscle and kidney.

T4 Is the less active form

Its half life is about seven days.
It is produced in much greater amounts than T3

Control of the thyroid

Production of T3 and T4 is controlled through a negative feedback cycle (see left) however, dopamine, somatostatin and glucocorticoids may reduce plasma TSH and so affect TSH release.

TRH = Thyrotropin Releasing Hormone, secreted by the hypothalamus
TSH = Thyroid Stimulating Hormone, secreted by the anterior pituitary

Both of these are peptide hormones.

Created February 2010