The document discusses different categories of hormones including peptides, amines, and steroids. Peptides are hydrophilic and act through second messengers like cAMP or calcium. Amines are derived from the amino acid tyrosine. Steroids are lipophilic and act through receptors in the cytoplasm. The plasma concentration of hormones depends on their secretion rate and negative feedback mechanisms. Endocrine disorders can involve excess or deficiency of hormones or decreased receptor responsiveness. The endocrine system differs from the nervous system in using chemical signals over distance versus direct wiring between cells.