Final answer:
T3 and T4 are thyroid hormones regulating the body's metabolism. T4 is converted to T3, which is more potent, affecting energy use, heat production, and heart rate. Their production is regulated by TSH and a negative feedback loop involving iodine, peroxidase enzymes, and thyroglobulin.
Step-by-step explanation:
Thyroid Hormones: T4 and T3
The thyroid gland produces two main hormones crucial for the regulation of metabolism: thyroxine (T4) and triiodothyronine (T3). T4, which contains four iodine ions, is the less active precursor to T3 and makes up about 90% of circulating thyroid hormones. On the other hand, T3 is the active form, more potent despite being only 10% of the circulating hormones, as T4 is converted to T3 in the target tissues. Both hormones play a key role in regulating gene expression, protein synthesis, and the body's basal metabolic rate. They increase metabolism in cells, which in turn boosts energy usage and heat production, apart from enhancing the rate and force of the heartbeat and increasing cellular sensitivity to adrenaline.
Regulation of T3 and T4
The production of T3 and T4 is stimulated by the thyroid-stimulating hormone (TSH), which is secreted by the anterior pituitary gland. Upon the binding of TSH to thyroid follicle receptors, T4 and T3 are synthesized from thyroglobulin with the help of iodine and a peroxidase enzyme. Although T4 is released in higher quantities, T3 is responsible for most thyroid hormonal activities, prompting the conversion of T4 to T3 within body tissues. The hormones are mostly bound to transport proteins in the bloodstream to maintain a reserve and released when needed. The feedback mechanism involving T3 and T4 levels regulates the release of TSH, ensuring a balanced thyroid hormone level in the body.
Functions and Effects of T3 and T4
While only a small portion of T3 and T4 is unbound and free to cross cell membranes, the majority is bound to transport proteins like thyroxine-binding globulins (TBGs). T3's potency is greater than T4's, and it's readily taken up by cells to exert its metabolic effects, such as mitochondrial ATP production and heat generation. The conversion of T4 to T3 in cells ensures the active hormone is available to manage energy utilization effectively.