Endocrinology: Thyroid Gland and Hormones

Thyroid Gland

• One of the largest endocrine glands, highly vascular, and the only gland that stores hormone.
• Composed of follicles with follicular cells surrounding thyroglobulin/thyroid hormones.
• Iodine and tyrosine are necessary for the production of T3 and T4.
• T3 and T4 increase the rate of glucose, fat, and protein metabolism in many tissues, thus increasing body temperature and promoting normal growth of many tissues.

ACTH

• ACTH (Adrenocorticotrophic hormone) is released by the anterior pituitary.
• ACTH causes cortisol secretion from the adrenal cortex (a glucocorticoid).
• ACTH causes aldosterone secretion from the adrenal cortex (a mineralocorticoid).
• ACTH also causes androgen (a weak sex hormone) secretion.

Adrenal Medulla

• Epinephrine and norepinephrine are secreted by the adrenal medulla in response to stress, physical activity, and low blood glucose levels.
• These hormones increase heart rate and force of contraction and cause blood vessels to constrict in skin, kidneys, gastrointestinal tract, and other viscera.

Hypothalamic Hormones

• Growth hormone-releasing hormone (GHRH) causes increased secretion of GH.
• Growth hormone-inhibiting hormone (GHIH) causes decreased secretion of GH.
• Thyrotropin-releasing hormone (TRH) causes TSH secretion.
• Melanocyte-releasing hormone (MRH) causes MSH secretion.
• Corticotropin-releasing hormone (CRH) causes ACTH secretion.
• Gonadotropin-releasing hormone (GnRH) causes secretion of gonadotropins LH and FSH.
• Prolactin-releasing hormone (PRH) causes increased prolactin secretion.
• Dopamine (prolactin-inhibiting hormone, PIH) causes decreased prolactin secretion.

Anterior Pituitary Hormones

• Growth hormone (GH) acts on most cells of the body, regulating overall metabolism and growth.
• Thyroid-stimulating hormone (TSH) stimulates the thyroid to secrete T3 and T4.
• Adrenocorticotropic hormone (ACTH) stimulates the adrenal cortex to secrete cortisol and aldosterone.
• Oxytocin stimulates uterine contractions during birth and ejection of milk from lactating breast (letdown reflex).

Control of Hormone Secretion

• Negative feedback: the hormone from the target endocrine cell travels to the hypothalamus and anterior pituitary, decreasing secretion of the tropic hormone.
• Positive feedback: the hormone from the target endocrine cell travels to the hypothalamus and anterior pituitary, increasing secretion of the tropic hormone.

Growth Hormone

• Stimulates uptake of amino acids and protein synthesis.
• Stimulates breakdown of fats to be used as an energy source.
• Promotes bone and cartilage growth.
• Regulates blood levels of nutrients after a meal.
• GH stimulates liver and skeletal muscle to make IGF-1.