QS Study

Mechanism of Thyroid-stimulating Hormone (TSH) secretion

Thyroid hormones are two hormones produced and released by the thyroid gland, namely triiodothyronine (T3) and thyroxine (T4). Thyroid-stimulating hormone is secreted from cells in the anterior pituitary called thyrotrophs, finds its receptors on epithelial cells in the thyroid gland, and stimulates that gland to synthesize and release thyroid hormones.

Thyroid-stimulating Hormone (TSH) secretion –

At a glance:

Nature: Glycoprotein,

Subunits: α and β

(α – similar to LH, FSH, HCG; β – Provides functional Specificity).


  • Half-life: 60 min,
  • Degradation site: Kidney, Liver.
  • Secretion: pulsatile, starts to rise at 9 pm, peaks at midnight, declines during the day.
  • Rate: 110 μg/day.
  • Plasma level (avg): 2 μU/ml.

Fig: Mechanism of Thyroid-stimulating Hormone (TSH) secretion

Mechanisms in TSH Secretion –

Control by TRH

Ant pit secretion of TSH a controlled by a hypothalamic hormone TRH. TRH directly affects thyrotropes of ant. pit. to increase an output of TSH.

TRH follows the following route

The median eminence of hypothalamus → hypothalamic-hypophyseal portal blood →Thyrotropes.

When the portal system from the hypothalamus to the ant. pit. gland is blocked, the rate of secretion by ant. pit. gland is greatly decreased but not reduced to zero.


TRH binds with the receptors in pit cell membrane → Activation of phospholipase second messenger → phospholipase C → activation of many 2nd messenger system → TSH release.

Other effects:

  • Exposure of cold → (+) Hypothalamus →↑ TRH →↑ TSH.
  • Excitement and anxiety → (-) Hypothalamus →↓ TRH →↓ TSH.

Negative feedback control by thyroid hormones:

  • ↑ T3, T4 in body fluids → ↓ TSH secretion
  • ↓ T3, T4 in body fluids → ↑ TSH secretion.

When conc. of T3, T4 reaches 1.75 times normal, the rate of secretion of TSH falls essentially to zero.

Function of TSH

  1. Increased proteolysis of the thyroglobulin.
  2. Increased activity of the iodide pump.
  3. Increased size and increased secretory activity of the thyroid cells.
  4. An increased secretory activity of thyroid cells.
  5. Increased number of thyroid cells and change from cuboidal to columnar cells and many infoldings of the thyroid epithelium into the follicles.