Hormones That Affect Metabolism 6.6 and D.5

3 Glands Affect Metabolism Thyroid gland Parathyroid gland Anterior pituitary

THYROID GLAND

Thyroid Gland Located at the base of the neck in front of the trachea Makes 2 important hormones: thyroxin (T4) and triiodothyronine (T3) They regulate body metabolism, growth, and differentiation of tissues, thermoregulation (Thyroid also makes hormone calcitonin – which acts on bone cells to lower the amount of calcium in the blood)

Thyroxine (T4) – 4 iodines

Triiodothyronine (T3)

Thyroxin and Metabolism Target cells: almost all cells in the body, however, its main targets are the most metabolically active ones such as liver, muscle, and brain Higher metabolic rates support more protein synthesis and growth and increases generation of body heat. Thyroxin secretion  rise in body temperature Thus, when you are cold thryoxin is secreted to stimulate heat production and raise body temperature.

Hyperthyroidism People who secrete higher levels of thyroxin will breakdown sugars and other nutrients at a faster rate. For these individuals, ~60% of glucose oxidized is released as heat 40% is converted into ATP and is usually consumed during daily activity. Thus, these individuals don’t gain weight (easily)

Hypothyroidism In contrast, individuals who produce lower levels of thyroxine do not break down sugars as quickly. Excess blood sugar is converted into glycogen, and the extra sugar into fat. The slower blood sugar is used, the faster fat is stored. NOTE: not all weight gain is due to hypothyroidism – in many cases, it’s poor diet.

Hypothyroidism Symptoms Lack of energy, fatigue Forgetfulness and depression Weight gain (despite lack of appetite) Feeling cold all the time Constipation (contraction of the intestines slowed down) Impaired brain development in children Symptoms indicate how important thyroxin is to regular functioning

Negative Feedback control When metabolic rate decreases (b/c low thyroxine) the hypothalamus is activated. The hypothalamus releases thyroid-releasing hormone to the anterior pituitary which causes the release of thyroid-stimulating hormone by the pituitary. TSH goes to the thyroid and signals the release of thyroxine. High levels of thyroxine inhibits the release of TRH.

Thyroid Disorders - Goiter Iodine, obtained from one’s diet, is an important component of T3 and T4 When not enough iodine is obtained from one’s diet, the thyroid enlarges to produce a goiter Lack of iodine→ decrease in T3/4 → TRH → TSH → thyroid stimulation – but with no hormones being produced, no negative feedback to shut off the thyroid stimulation so thyroid enlarges

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Foods High in Iodine Milk Seaweed cod, Iodized salt Shrimp Baked potatoes Navy beans Tuna fish strawberries, bananas

Parathyroid Glands

Parathyroid Glands 4 small glands within the thyroid gland Usually nerves or other hormones regulate the endocrine glands but the parathyroid glands respond directly to chemical changes in their immediate surroundings. Produces parathyroid hormone (PTH) which regulates blood calcium levels

Low calcium PTH release kidneys and intestines calcium is released by bones retain absorb (retain) calcium When calcium levels reach normal levels again, the parathyroid glands are inhibited. Abnormally high levels of PTH would cause prolonged breakdown of bones

Parathyroid gland PTH also helps activate vitamin D Vitamin D is produced by skins cells from exposure to UV light (ie sunlight) and is an important component of several hormones Vitamin D is required for calcium absorption from food in the intestines. Teenagers and adults likely spend enough time outside to make enough vitamin D. Children, pregnant women, the elderly and people who do not get enough UV light will need to supplement the amount made by their skin. Dietary sources of Vitamin D: oily fishes (tuna, sardines, mackrel), eggs, liver, and fortified milk and margarine

Vitamin D Deficiency p668 Low levels of vitamin D can lead to insufficient absorption of calcium and phosphate from food leaving to the disease rickets – when the bones develop improperly,

UV light and Vitamin D There are some harmful consequences to UV light such as mutations causing skin cancer. Melanin, the skin pigment absorbs UV light Thus individuals with darker skin (more melanin) have better protection against skin cancer, however, they have reduced vitamin D synthesis.

Anterior Pituitary Gland Secretes growth hormone – also known as somatotropin. GH stimulates growth factors produced by the liver which stimulate cell division in growth plates causing the elongation of the skeleton. Also stimulates protein synthesis by increasing the amount of amino acids uptake by cells and stimulating ribosomes. It also causes an increase in muscle mass As a person ages, GH production declines and cellular repair and protein replacement decreases. Protein is replaced by fat causing changes in body shape.

GH Low secretion of GH during childhood can result in dwarfism High secretion cause result in gigantism Effects most body cells, but most particularly bone and cartilage cells. The binding of GH stimulates the release of insulin-like growth factors which will stimulate bone and cartilage growth If GH production continues after cartilaginous growth plates have been fused, bones will no longer continue to increase in length but now in width. Typically jaw, forehead, finger, and toe bones will increase in width Disorder is called acromegaly – the broadening of facial features

Acromegaly

GH and sport Since growth hormones promotes an increase in muscle mass, some athletes take growth hormone to build muscles The availability of growth hormones has increased due to the development of genetically modified organisms that can produce it in large quantities Because there is a correlation between muscle size and strength, competitors in sports that require short bursts of explosive strength would benefit. It potentially also allows for tired muscles to recover more quickly allowing an individual to train harder and more often.

Leptin A protein hormone secreted by adipose tissue (fat storage cells) It acts on the hypothalamus in the brain and controls appetite. If adipose tissue increase blood leptin concentration increase long-term appetite inhibition and reduced food intake

Leptin

Leptin Experiments Both mice do not produce leptin and thus overeat. The mouse on the RIGHT was injected with leptin. Its appetite declined, its energy metabolism increased, body mass dropped by 30% in one month

Can leptin injections cure obesity in humans? A double-blind study was conducted with 73 obese (human) volunteers Volunteers injected themselves with either varying doses of leptin or a placebo. Results: The leptin injects inducted skin irritation and swelling; only 47 patients completed the trial.

While the patients receiving the high dosage of leptin lost avg of 7 While the patients receiving the high dosage of leptin lost avg of 7.2 kg (compared to 1.3kg of the placebo injectors), there was a large degree of variation (from a loss of 15kg to a gain of 5 kg) Conclusion: Human physiology is different from mice. In fact, it is found that most obese humans have exceptionally high blood leptin concentrations. Hypothalamus seems to have become resistant to leptin. In cases where obesity is due to a mutation in the leptin gene, individuals respond positively to leptin injections. However, leptin has a short life and must be injected several times a day.