The thyroid gland is located in the lower part of the neck and is partially wrapped around the trachea (windpipe). It has two lobes that are joined together in the middle, giving it a shape similar to a butterfly.

Basic Anatomy

The four parathyroid glands are located in the back of the thyroid gland. They are roughly the size of a pea and yellow in color.

Posterior (Back) View of Thyroid Gland

The Thyroid Gland

The thyroid gland regulates the body’s metabolism by adjusting the rate at which glucose is oxidized (through cellular respiration).

The main function of the thyroid gland is to produce hormones. These hormones regulate how much energy the body expends (called metabolism).

The Thyroid Gland and HomeostasisThe thyroid gland secretes two hormones that help controlmetabolism. They are called Thyroxine (T4) andTriiodothyronine (T3). Iodine is an essential component ofboth T3 and T4.

Thyroxine (T4) is secreted by the thyroid gland. It is theinactive form of Triiodothyronine (T3). It is easily convertedby body cells into T3 by removing one of its four iodinemolecules.

Triiodothyronine (T3) raises metabolism by stimulating increased sugar utilization by cells. T3 is 8 times moreeffective than T4 at increasing metabolism.

T4 - Thyroxine

T3 -Triiodothyroxinine

Iodine

The Parathyroid Glands and Homeostasis

The parathyroid glands help regulate calcium levels in the blood and lowerphosphate levels. Calcium is used by the body for development andmaintenance of strong bones and teeth. It is also helps the heart, nerves,muscles and other body systems to work properly.

When the level of calcium in the blood is too low, the parathyroid glandsrelease parathyroid hormone (PTH). A rise in PTH causes calcium levels inthe blood to increase and phosphate levels to decrease. This hormoneincreases the level of calcium by influencing the kidneys, intestines, andbones.

PTH causes the kidneys to reabsorb more calcium, so less calcium is excreted in the urine. The intestines absorb and retain more calcium

from undigested foods. Calcium is released from bones where it is stored.(98% of calcium is stored in the skeletal system.)

When bone cells break down, calcium is separated from phosphate ions.

The calcium is reabsorbed into the blood and the phosphate ions are excreted in the urine. Calcium levels in the blood return to normal, so PTH secretion stops.

Kidneys reabsorb Ca2+ Intestines absorb Ca2+ Bones release Ca2+

http://www.nlm.nih.gov/medlineplus/tutorials/thyroidsurgery/htm/_no_50_no_0.htm

Parathyroid Hormone Feedback Loop

1. Low Ca2+ levels in the blood cause the parathyroid glands to release parathyroid hormone (PTH).

2. The kidneys reabsorb more calcium, the intestines absorb and retain calcium from food, and calcium is released from bones.3. These 3 factors cause Ca2+ levels in the blood to rise.4. Calcium levels are back to normal so PTH secretion stops.

Important Hormones

Hormone Secreted by… Target Area(s) Effect

Thyroxine (T4) &Triiodothronine (T3)

Thyroid All body cellsControls

metabolism

Calcitonin Thyroid Bone cellsLowers the level of calcium in the

blood

Parathyroid Hormone (PTH)

ParathyroidKidneysIntestinesBones

Raises calcium levels in the

blood

We already know about feedback loops in Homeostasis…

Monitors/ Receptors

Control System

Effectors/Regulators

Pituitary

Hypothalamus (senses decreased metabolism)

Thyroid

Thyroid –releasing hormone (TRH)

Thyroid Stimulating Hormone (TSH)

T3 and T4

Negative Feedback Loop for the Thyroid Gland

Metabolism is back to normal, hormone secretion slows down

T3 and T4 Feedback Loop

Legend

TRH = Thyroid Releasing Hormone

TSH = Thyroid Stimulating Hormone

T3 = Triiodothyronine

T4 = Thyroxine

GoitersA goiter is a pronounced swelling of the thyroid gland in the neck caused by insufficient iodine intake in the diet.

The main source of iodine in your diet is iodized salt.

Hypothyroidism(“Hypo” means less)

Underactive Thyroid Gland

Occurs when the thyroid doesn’t produce enough hormones.

Hypothyroidism decreases metabolism.

Hyperthyroidism(“Hyper” means more)

Overactive Thyroid Gland

Occurs when the thyroid produces excess hormones.

Hyperthyroidism increases metabolism.

Thyroid Disorders

Hypothyroidism

Hyperthyroidism

• fatigue • feeling cold• weight gain • depression • reduced concentration • swollen face • dry skin• dry & brittle hair and nails• constipation • goiter

• fatigue• feeling hot• weight loss• increased appetite• nervousness and anxiety• increased heart rate• increased blood pressure• shaking hands • frequent bowel movements• bulging eyes • hair loss

Treatment Hypothyroidism: most patients take hormone replacement pills (i.e. eltroxin) to bring their thyroid hormone levels back up to normal.

Hyperthyroidism: patients are treated with drugs that bind to iodine to partially block the production of hormones, bringing hormone levels back down to normal. However, if medication isn’t working radioactive iodine can be used to destroy the hyperactive thyroid tissue. This can sometimes affect the entire thyroid gland leading to hypothyroidism.

Sometimes surgery is required to remove tumors or cysts that could cause either hypothyroidism or hyperthyroidism.

http://video.about.com/thyroid/Thyroid-Disease.htm

QUIZ

1. True or False?

A parathyroid gland is larger than a thyroid gland.

FALSE

2. How many parathyroid glands does a typical person have?

A) OneB) TwoC) FourD) Six

Answer: C

3. True or False?

The thyroid gland plays an important part in controlling metabolism.

TRUE

4. What is hypothyroidism?

A)The thyroid gland is incapable of making hormones.

B)The thyroid gland produces too many hormones.

C)The thyroid gland produces not enough hormones.

D)The parathyroid glands are cancerous.

Answer: C

5. True or False?

Hyperthyroidism causes the body cells to burn more energy than normal, decreasing metabolism.

FALSE

6. What causes a goiter?

A) HypoparathyroidismB) HyperparathyroidismC) An insufficient amount of iodine in your dietD) A genetic mutation

Answer: C

7. You are a doctor. A patient has inexplicably lost a lot of weight, despite having an increased appetite. They have an increased heart rate and high blood pressure as well. They also feel very tired, and have bulging eyes.

What would you diagnose them with?

Hyperthyroidism