The endocrine system consists of multiple organs that secrete hormones directly into the bloodstream to control body processes and maintain homeostasis, the state in which the body’s hormones and nutrient levels are balanced. Hormones then travel through the bloodstream and bind to receptors on their target, triggering a response by the target tissue. The two main classes of hormones are steroid and peptide hormones.
The body maintains homeostasis through a variety of feedback loops. In positive feedback, the response to a deviation will lead to a larger response. These are very rare and often lead to large physiological changes, such as delivery of a baby. Most feedback loops operate under negative feedback, a process in which a deviation from normal is detected, a response is made, and the response alleviates need for further action to address the deviation. An example of negative feedback is shown in the figure.
Here is how an expert would approach a question on feedback loops.
| Question | Analysis |
| Which of the following is an example of a negative feedback loop? | Step 1: This question is asking for an example of a negative feedback loop. The answers refer to a drop in hormone levels, so you can be more specific with your prediction. |
| Step 2: Recall that in negative feedback, a deviation from homeostasis is detected, a response is made, and the response alleviates the need for further action. | |
| Step 3: Predict that a decrease in hormone level will lead to the production of more hormone. Once hormone levels stabilize, no more hormone will be released. | |
| (A) A decrease in thyroid hormone levels is detected, and the thyroid produces more hormones. Following the increase in hormone levels, the thyroid produces more hormones. | Step 4: A decrease in thyroid hormone would cause more to be released. However, once levels stabilized, the thyroid would release less, not more, hormone. Eliminate. |
| (B) A decrease in thyroid hormone levels is detected, and the thyroid produces less hormones. Following the decrease in hormone levels, the thyroid produces less hormones. | A decrease in thyroid hormone would lead to more, not less, hormone release. Eliminate. |
| (C) A decrease in thyroid hormone levels is detected, and the thyroid produces more hormones. Following the increase in hormone levels, the thyroid produces less hormones. | This is a perfect match for your prediction. Choose choice (C). |
| (D) A decrease in thyroid hormone levels is detected, and the thyroid produces less hormones. Following the increase in hormone levels, the thyroid produces more hormones. | A decrease in thyroid hormone would lead to more, not less, hormone release. Eliminate. |
The master regulatory gland of the endocrine system is the hypothalamus, which is located in the brain under the thalamus. It serves as the link between the nervous system and the endocrine system by releasing hormones that control other endocrine glands in response to different stimuli. The hypothalamus also produces two hormones that are stored and released by the posterior pituitary:
The hypothalamus also releases hormones to induce hormone release from the anterior pituitary. The anterior pituitary releases both direct hormones, which stimulate a response directly at a target organ, and tropic hormones, which induce hormone release by other glands. Following is a summary of the seven hormones released by the anterior pituitary and their functions:
A great mnemonic to remember these hormones is FLAT PEG, where the FLAT hormones are the topic hormones and the PEG hormones are the direct hormones.
In addition to the hypothalamus and pituitary gland, many other endocrine glands are either regulated by the hypothalamus or respond directly to physiological changes.
The pineal gland, located in the center of the brain, secretes melatonin to regulate sleep cycles. The release of melatonin is governed by light–dark cycles. Melatonin supplements are readily available at drugstores and can be used to induce sleep.
The thyroid and parathyroid are located in the neck and control both metabolism and calcium balance. The parathyroid gland releases parathyroid hormone in response to low blood calcium. It induces the breakdown of bone to increase blood calcium levels. The thyroid gland, upon stimulation by TSH, will absorb iodine and produce thyroid hormones. Thyroid hormones (T3 and T4) regulate metabolic rate. The thyroid also releases calcitonin in response to high blood calcium levels and induces the storage of calcium in bone. It "tones down" blood calcium.
Thyroid diseases include hypothyroidism and hyperthyroidism. Hypothyroidism is caused by insufficient thyroid hormone production, and its symptoms include weight gain, fatigue, and cold intolerance. At the other end of the spectrum is hyperthyroidism, caused by overproduction of thyroid hormones. Symptoms include weight loss, hyperactivity, and heat intolerance. Additionally, although uncommon in developed countries, insufficient iodine intake can lead to a significant enlargement of the thyroid known as a goiter. The widespread availability of iodized salt has greatly reduced the prevalence of goiters.
The pancreas is responsible for regulating blood glucose levels. It releases two main hormones:
The most common endocrine disease of the pancreas is diabetes, caused by insufficient insulin secretion, which leads to abnormally high blood glucose levels. There are two types of diabetes. Type 1 diabetes is caused by damage to the cells that produce insulin and must be treated with administration of exogenous insulin. Type 2 diabetes is caused by insulin resistance: the body still produces insulin, but the cells are unable to respond. Type 2 diabetes can often be managed with diet and lifestyle changes.
The adrenal glands are located just above the kidneys. They are composed of an outer cortex and inner medulla, similar to the kidney. The adrenal medulla secretes epinephrine, which induces the "fight or flight" response. The adrenal cortex secretes hormones that regulate salt and sugar balance in the body, including the hormone aldosterone, which regulates salt balance and is discussed in greater detail in Lesson 7: The Genitourinary System.
Sex hormones are released by the testes and ovaries. These hormones are steroid hormones and thus have long-lasting effects on the body. Estrogen is produced by the ovaries and leads to the development of secondary female sex characteristics. Progesterone is also released by the ovaries and maintains the uterine wall. These two hormones are discussed in greater detail in Lesson 9: The Reproductive System in relation to the menstrual cycle. The testes produce testosterone, which induces male sexual differentiation in utero and the development of secondary male sex characteristics during puberty.
Now you try a question testing the endocrine system.
Which of the following hormones would be secreted following a drop in blood calcium levels?
Step 1: This question is asking which hormone is released when blood calcium levels are too low.
Step 2: Recall that calcitonin and parathyroid hormone act antagonistically to regulate blood calcium levels. Parathyroid hormone is released when blood calcium is too low, while calcitonin is released when blood calcium is too high.
Step 3: Predict that parathyroid hormone would be released due to low blood calcium.
Step 4: Choice (C) matches this prediction. Epinephrine, (A), is released by the adrenal cortex to initiate the "flight or fight" response. Calcitonin, (B), is released when blood calcium is high, and insulin, (D), regulates blood glucose, not calcium.