Mechanism of Action of Steroid Hormones: Animation

Hormones are the chemicals produced within the body by some specialized cells and have specific effects on the activity of target organs through specific receptors. There are 2 types of hormones:

a) Lipid soluble:

1. Can pass through cell membrane
2. Act via intracellular and intranuclear receptors
3. Mechanism: Protein synthesis
4. eg. Steroid hormones and thyroid hormones

b) Water soluble:

1. Cannot pass through cell membrane
2. Act via membrane receptors
3. Mechanism: cAMP, cGMP, IP3, Calcium-calmodulin, Tyrosine kinase
4. eg. All other hormones except steroid and thyroid hormones

Before Moving on to the mechanism of action, we will list the major steroid and thyroid hormones.

Steroid hormones (Cholesterol derivatives):

1. Glucocorticoid (Cortisol)
2. Estrogen
3. Testosterone
4. Progesterone
5. Aldosterone
6. Vitamin D (Calcitriol)

Thyroid hormones (Amine derivatives):

1. Tri-iodothyronine (T3)
2. Thyroxine (T4)

Mechanism of Action of Steroid Hormones:

1. Simple Diffusion: The lipid soluble hormones diffuses through the cell membrane to enter the cell.
2. Hormone binds to the intracellular receptor composed of a "Hormone binding" domain, a "DNA binding" domain and a "amino terminal" which interacts with other transcription factors. Binding of the hormone leads to exposure of DNA binding zone.
3. Hormone-receptor complex enters nucleus and dimerizes.
4. Binding to HREs: Hormone-recpetor dimers bind to Hormone (Steroid) Receptor Elements (SREs or HREs) of DNA.
5. Transcription: DNA transcription leads to formation of mRNA.
6. Translation: mRNA undergoes translation to produce new proteins. e.g. Calbindin for Vitamin D
7. Physiologic action of hormones.

What is the difference in the mechanism of action of thyroid and steroid hormones?

After passing through the cell membrane, steroid hormones except calcitriol bind to the intracellular receptor in the cytosol before entering the nucleus while the thyroid hormones and calcitriol directly enter the nucleus to bind to the intranuclear receptor. Beside, this difference all other steps are similar for both the hormones.

Summary: