Thyroid hormones diffuse through the plasma membrane of a target cell and enter the nucleus of the
cell. Once in the nucleus, the thyroid hormone binds to a specific receptor for the hormone located in
the nucleus, forming a thyroid hormone-receptor complex. This complex then binds to a specific
region of DNA known as responsive element. These responsive elements are unique for the different
hormone-receptor complexes that can form. Binding of the thyroid hormone-receptor complex to the
responsive element results in an increase in transcription of specific genes. The increase in gene
transcription results in an increase in the amount of mRNA for the gene in the cytoplasm of the cell.
This mRNA will be translated in the cytoplasm resulting in an increase in the amount of protein in the
cell. Increasing the cellular concentration of protein will bring about the cellular responses specific for
thyroid hormone in that specific cell.
What is the normal role(s) of thyroxin (T4) and triiodotyronine (T3) on the body?
Why would an
imbalance in thyroid hormones have such widespread effects on the body?