Summary
Highlights
The hypothalamus releases thyrotropin-releasing hormone (TRH) from the paraventricular nucleus, which travels through the hypophysial portal system to the anterior pituitary. There, thyrotropes respond to TRH by secreting thyroid-stimulating hormone (TSH). TSH then circulates to the thyroid gland, stimulating the production of T3 and T4 (thyroid hormones).
High levels of T3 and T4 exert a negative feedback mechanism on the hypothalamus, inhibiting TRH production. This leads to decreased TSH and subsequently lower T3 and T4 levels. Conversely, low T3 and T4 levels stimulate the paraventricular nucleus to produce more TRH, leading to increased TSH and thyroid hormone production.
Thyroid hormones promote normal bone growth and maturation. They also support normal muscular system development and function.
T3 and T4 increase the basal metabolic rate by enhancing sodium-potassium ATPase activity, leading to increased oxygen usage. They also stimulate metabolic pathways such as lipolysis, glycolysis, and gluconeogenesis, making them hyperglycemic hormones. Additionally, they increase LDL uptake, helping to lower plasma cholesterol levels.
Thyroid hormones promote normal cardiac output, nerve development in the brain by increasing synapses, myelination, and dendrites. They also promote normal GI motility and secretions, and maintain normal hydration of the skin.
In summary, thyroid hormone is regulated by TRH and TSH and plays a crucial role in bone and muscle growth, metabolic rate, lipid breakdown, cardiac function, nervous system development, GI motility, and skin hydration. Its levels are tightly controlled by a negative feedback loop to maintain homeostasis.