Endocrine System Part 2

Thyroid The thyroid is a gland in your neck, stimulated by the anterior pituitary’s release of TSH The thyroid releases thyroxine which stimulates cells to increase metabolism Thyroxine’s key ingredient is iodine, which is required in small doses for life – Hypothyroidism (less thyroxine) causes exhaustion and weight gain – Lack of iodine can cause hypothyroidism and a swelling of the thyroid (goiter)

Protip: do NOT OD on iodine Common reaction to discovery of metabolic properties of iodine: “Wow awesome I can take iodine and it will make me burn more calories!” – Yes, but… Excessive intake of iodine can result in a thyroid storm, where all the internal organs are also in overdrive – Diet-based hyperthyroidism – Results in severe illness and possible death (and yes, weight loss)

Calcium Management The thyroid releases calcitonin, ostensibly to decrease calcium levels – Not clear whether its action is important The thyroid also has small glands attached called the parathyroid glands – Four or eight small dots on the back The parathyroid glands release parathyroid hormone or PTH

PTH PTH is released when blood calcium is low – Stimulates digestive system to absorb calcium – Stimulates kidneys to reabsorb calcium from urine – Stimulates osteoclasts to release calcium from bone This is a big part of why calcium intake should be ongoing and not just once a week

Glucose Management Long-term metabolism is controlled by the pituitary gland (GH) Medium-term metabolism is controlled by the thyroid (thyroxine) Short-term metabolism is controlled by the pancreas – Releases hormones that manage glucose levels in the blood

Why Glucose? Recall that glucose is the energy source for all cells in the body – Neurons in particular absolutely must be nourished by glucose Also recall that glucose is a polar (hydrophilic) molecule and so is “sticky” – No one wants to have their heart pump syrup through their arteries

Insulin The pancreas releases insulin whenever blood glucose gets high – Signals all cells to absorb additional glucose This helps cells use and store energy – The liver produces more glycogen – Fat cells convert sugar to fat If cells cannot absorb sugar then they cannot produce ATP and make things/move things – What do we call it when there is not an insulin signal to the cell?

Insulin and Diabetes Failure to release or receive insulin is called diabetes -Excessive sugar in the blood is called hyperglycemia and often results from diabetes Type I diabetes results from a lack of insulin (usually damage to the pancreas by the immune system) Type II diabetes results from excessive sugar intake, where cells become resistant to insulin signals via overuse

Glucagon The pancreas releases glucagon whenever blood sugar gets low – Stimulates fat and liver to release glucose into bloodstream A deficiency of glucagon or excess of insulin can cause hypoglycemia – Critically low blood sugar causes crankiness followed by coma and death

Gonads The testes in men and ovaries in women are collectively referred to as gonads – AKA “the ‘nads” – Produce gametes (sperm & eggs) for babymaking Produce sex hormones testosterone, estrogen and progesterone – Stimulated to do so by FSH and LH from the anterior pituitary

Effects of Sex Hormones Sex hormones promote production of gametes and development of gonads Sex hormones also promote secondary sexual characteristics, especially after puberty – Men: facial/chest hair, Adam’s apple – Women: Breast development, wider hips – Men and women: pubic and axillary hair, production of sebum, actual interest in sex What part of the brain do you think the sex hormones affect?

Bonus Hormones Erythropoietin (EPO) is a hormone released by the kidneys in response to low blood oxygen – Increases production of red blood cells Two hunger hormones act against each other: – Leptin is produced by fat cells and lowers hunger – Ghrelin is produced by the intestines that stimulates hunger (mimicked by marijuana) Human Chorionic Gonadotropin (hCG): Pregnancy hormone produced by the placenta that blocks menstruation – Tested for in pregnancy test

Negative Feedback Loops! Body temperature falls: TSH increased, thyroxine increased, body temperature increases, TSH inhibited Glucose goes high: insulin released, glucose goes down, insulin inhibited Glucose goes down: glucagon released, glucose goes up, glucagon inhibited Calcium goes down: PTH released, calcium goes up, PTH inhibited Also note that injecting steroids into the body will register as testosterone levels, causing an excess to cause the testes to shrivel in response