Hormone levels are regulated by four different mechanisms:
- Hormone levels: feedback to pituitary and hypothalamus (thyroid hormones)
- Hormone effects: feedback to the CNS (stress and epinephrine)
- Levels of circulating substance: feedback to a specific endocrine gland (glucose and insulin)
- Circadian rhythms: hormone release follows a daily pattern (cortisol peaks mid-morning)
Thyroid hormone levels are maintained constant by feedback to the anterior pituitary gland and to the hypothalamus. Proper regulation of thyroid hormone release can be judged by measuring the levels of thyroid hormones (T3 and T4) in the blood.
In stress, epinephrine is released by the adrenal medulla, which is under sympathetiic control from the hypothalamus. So long as stress continues, epinephrine will be released. Its release is regulated by removing the stressor, which reduces sympathetic activation and stops production of epinephrine.
Glucose levels are maintained relatively constant by the response of the pancreas to blood glucose. When glucose levels are elevated after a meal, insulin is released, which promotes glucose uptake into cells. This reduces blood glucose levels and shuts down insulin secretion.
Circadian rhythms are daily cycles during which the levels of hormones (and other biological activities) change throughout the day. Plasma cortisol levels show a circadian rhythm; they peak mid-morning and decrease at night. This is not the only mechanism that regulates cortisol levels, as they also respond to stress, prompting the body to increase its energy production in the face of a continuing threat.
QBReview: Review this Lesson
Making a Difference in the World 