Final answer:
Thyroid hormone receptors possess a ligand-binding and dimerization domain crucial for hormone binding and gene expression. They can bind DNA even without hormone presence to modulate transcription, and these receptors exist in various isoforms due to gene splicing. They play a wide-reaching role in metabolic regulation across the body.
Step-by-step explanation:
Thyroid hormone receptors are vital components of physiological regulation and exhibit several critical properties related to hormone binding and gene expression. Like other members of the nuclear receptor superfamily, these receptors consist of three functional domains. A key feature is the ligand-binding and dimerization domain at the carboxy-terminus, which can undergo mutations leading to thyroid hormone resistance syndromes. Such disorders are typically manifested clinically by symptoms like goiter, altered serum concentrations of thyroid hormones, and can even lead to attention deficits.
These receptors are also capable of DNA binding in the absence of hormones, bringing about transcriptional repression. The binding of the hormone then induces a conformational shift, toggling the receptor into a transcriptional activation mode. It is important to differentiate that thyroid hormone receptors bind to DNA that is already present within the cell nucleus, contrasting with the cytosol or nucleus binding seen with steroid hormones. This interaction triggers specific gene transcriptions that profoundly influence metabolic rates and protein synthesis.
Mammalian thyroid hormone receptors are expressed through the genes designated alpha and beta, which are alternatively spliced to yield different isoforms recognized as α-1, α-2, β-1, and β-2. These receptors are dispersed throughout the body in various tissues, emphasizing their system-wide role in metabolism. The intracellular hormone receptors for thyroid hormones leverage carrier-mediated mechanisms for cellular entry, which are sodium and energy-dependent, highlighting their meticulous functional regulation within the endocrine system.