Steroid and Thyroid Hormones

Cytoplasmic receptors for steroids are proteins with two subunits that bind to the steroid molecules. When both receptor sites are occupied by the steroid then the receptor-steroid complex migrates to the nucleus where one subunit ensures that the complex binds to the specific site on the chromatin. Then the receptor subunits separate and the other subunit interact directly with the adjacent region of DNA molecule resulting in the transcription of the DNA segment into mRNA.

Thyroid hormone acts in a similar manner except that the receptor is located in the nucleus. The major hormone secreted by thyroid is thyroxine (T₄). It travels in blood attached to carrier proteins. Thyroid also secretes a small amount of triiodothyronine (T₃). Carrier proteins have higher affinity for T₄ and very little T₄ is free in the plasma. Only the free T₄ and T₃ enter the cells. The rest of the bound T₄ acts as a reservoir for slow release. The free T₄ that enters the cell s also converted to T₃ enzymatically. Therefore, T₃ is the chemically more potent and active form of thyroid hormone.

Figure:Steroid and Thyroid Hormones