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[[Image:Adrenaline.png|thumb|Adrenaline - Chemical structure]]
[[Image:Adrenaline.png|thumb|Adrenaline - Chemical structure]]
A '''Hormone''' is a chemical messenger that signals to specific cells to change their behavior in specific ways. Hormones are secreted into the [[bloodstream]] by ductless endocrine [[gland]]s, which form part of the [[endocrine system]]. Hormones may be classified into three broad categories:
A '''hormone''' is a chemical messenger that signals to specific cells to change their behavior in specific ways. Hormones are secreted into the [[bloodstream]] by ductless endocrine [[gland]]s, which form part of the [[endocrine system]]. Hormones may be classified into three broad categories:
# [[Amines]]: Hormones which are derived from the amino acid tyrosine ([[Dopamine]], [[epinephrine]], [[norepinephrine]], [[thyroid]] hormones).
# [[Amines]]: Hormones which are derived from the amino acid tyrosine ([[Dopamine]], [[epinephrine]], [[norepinephrine]], [[thyroid]] hormones).