Thyroid hormones diffuse through the plasma membrane

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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.
3. 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?
4. What is PBI and why was it used as a test to indicate the activity of the thyroid gland? What are the newer blood tests used to monitor thyroid activity?
T4 TESTS T4 circulates in the blood in two forms:

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