Regulation of the thyroid

• hypothalamus: TSH-releasing hormone is produced, stimulating the pituitary to release TSH, which stimulates the thyroid gland to secrete thyroxine, T4

• only 20% of T3 is produced by the thyroid. it is 5-10x more potent than T4 because it binds with higher affinity to the thyroid hormone receptor

• the remaining 80% comes from peripheral conversion of T4 to T3

• thyroid binds to a nuclear receptor that directly regulates transcription of genes

Thyroid hormones

• basic structure comprises two benzene rings connected by an ether bond

• in T4, each benzene ring has two iodine substituents

• T3 is missing an iodine on the terminal benzene

• reverse T3 is missing an iodine from the other (non-terminal) benzene

Category: Endocrinology Notes