Hypothalamus, Pituitary and Thyroid Flashcards
The anatomy of the hypothalamus and pituitary
The hypothalamus is a small region located in the ventral part of the brain, just below the thalamus. It is composed of several nuclei that are responsible for controlling various autonomic and endocrine functions. The hypothalamus is connected to the pituitary gland via the hypothalamic-pituitary portal system, which allows the hypothalamus to secrete hormones that regulate the function of the pituitary gland.
The pituitary gland is divided into two parts: the anterior pituitary and the posterior pituitary. The anterior pituitary is composed of glandular tissue and is responsible for producing and secreting several hormones, including growth hormone, thyroid-stimulating hormone, adrenocorticotropic hormone, follicle-stimulating hormone, luteinizing hormone, and prolactin. The release of these hormones is regulated by releasing hormones and inhibiting hormones secreted by the hypothalamus.
The posterior pituitary is composed of nervous tissue and is responsible for storing and releasing two hormones: oxytocin and vasopressin. These hormones are produced in the hypothalamus and transported to the posterior pituitary for storage and release.
The main pituitary hormones and their actions
Growth hormone (GH): This hormone stimulates growth and cell reproduction in humans and other animals. It also has metabolic effects, such as increasing the breakdown of fats for energy and decreasing glucose uptake by cells.
Prolactin (PRL): This hormone is responsible for milk production in females after childbirth. In males, it may have a role in the immune system and reproductive function.
Adrenocorticotropic hormone (ACTH): This hormone stimulates the adrenal glands to produce cortisol, which regulates the body’s response to stress and helps to maintain blood pressure and blood sugar levels.
Thyroid-stimulating hormone (TSH): This hormone stimulates the thyroid gland to produce thyroid hormones, which regulate metabolism, body temperature, and other bodily functions.
Follicle-stimulating hormone (FSH): In females, FSH stimulates the growth of ovarian follicles and the production of estrogen. In males, it stimulates the production of sperm.
Luteinizing hormone (LH): In females, LH triggers ovulation and the production of progesterone. In males, it stimulates the production of testosterone.
The thyroid hormones and their actions
Metabolism: Thyroid hormones stimulate the metabolic rate of cells throughout the body, which affects the way that the body burns calories and uses energy.
Growth and development: Thyroid hormones are important for normal growth and development, especially in children. They also play a role in maintaining the health of tissues and organs throughout life.
Heart function: Thyroid hormones help to regulate heart rate and the strength of heart contractions, which are important for maintaining a healthy cardiovascular system.
Brain function: Thyroid hormones play a role in cognitive function, including memory and concentration. They also influence mood and emotional well-being.
Bone health: Thyroid hormones are important for maintaining bone health and preventing osteoporosis.
Digestion: Thyroid hormones stimulate digestive processes and help to maintain healthy digestion.
Menstrual function: Thyroid hormones are important for regulating menstrual cycles and fertility in women.
Body temperature: Thyroid hormones help to regulate body temperature, which is important for maintaining a healthy internal environment.
Control of thyroid function
The hypothalamus and pituitary gland are two key structures involved in the control of thyroid function. The hypothalamus produces thyrotropin-releasing hormone (TRH), which signals the pituitary gland to release thyroid-stimulating hormone (TSH). TSH then stimulates the thyroid gland to produce and release the thyroid hormones, T3 (triiodothyronine) and T4 (thyroxine).
The levels of T3 and T4 in the blood are tightly regulated by negative feedback mechanisms. When the levels of these hormones are too low, the hypothalamus and pituitary gland increase their production of TRH and TSH, respectively, which in turn stimulates the thyroid gland to produce more hormones. When the levels of T3 and T4 are too high, the production of TRH and TSH is reduced, leading to a decrease in the production of thyroid hormones.
In addition to this feedback loop, several other factors can also influence thyroid function, including stress, nutrition, and certain medications. For example, stress can lead to an increase in cortisol levels, which can interfere with the production and release of thyroid hormones. Similarly, certain medications, such as lithium and amiodarone, can affect thyroid function by interfering with the production or uptake of iodine, which is necessary for the production of thyroid hormones.
How is thyroid hormone secretion regulated?
Hypothalamus: The hypothalamus produces thyrotropin-releasing hormone (TRH) in response to low levels of thyroid hormones. TRH travels through the blood to the pituitary gland.
Pituitary gland: The pituitary gland responds to TRH by producing thyroid-stimulating hormone (TSH), which is released into the bloodstream. TSH then travels to the thyroid gland.
Thyroid gland: TSH stimulates the thyroid gland to produce and release T4 and T3. Most of the thyroid hormones produced are in the form of T4, which is converted to the more active form, T3, in the liver, kidney, and other tissues.
Feedback loop: As the levels of T4 and T3 in the bloodstream increase, they inhibit the production of TRH and TSH through a negative feedback loop. This reduces the stimulation of the thyroid gland, and consequently, the production and secretion of thyroid hormones.
Thyrotropin releasing hormone (TRH)
✓ Produced by hypothalamus
✓ Down-regulated by T4, T3
✓ Stimulates TSH formation
▪ Thyroid stimulating hormone (TSH)
✓ Produced by anterior pituitary
✓ Upregulated by TRH
✓ Downregulated by T4, T3
✓ Stimulates iodine uptake, colloid
endocytosis, number and secretory activity
of thyroid cells, and therefore, secretion of
T4, T3
