Hormones are chemical messengers released by glands into the blood. They reach almost every cell, but only cells with the right receptor respond.
Two kinds of hormone
Water-soluble hormones, such as insulin and epinephrine, bind receptors on the cell surface and act within seconds to minutes. Fat-soluble hormones, such as steroids and thyroid hormone, enter the cell and change gene activity over hours to days.
Negative feedback
Most hormones are controlled by negative feedback: a rising level turns off its own release. Thyroid hormone suppresses TSH from the pituitary, so TSH is high when the thyroid is underactive.
Hypothalamus and pituitary
The hypothalamus signals the pituitary, which in turn controls the thyroid, the adrenal glands and the gonads. This chain lets a small signal in the brain produce a large effect in the body.
Patterns over time
Some hormones follow a daily rhythm; cortisol peaks in the early morning. The time of a blood test can change how the result should be read.
At the bedside
When reading hormone tests, look at the pituitary hormone and its target hormone together, such as TSH with free T4. Ask about symptoms of too much or too little, like heat or cold intolerance and weight change.
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Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).