Human metabolism is regulated through both neural and endocrine mechanisms. Though these are two distinct mechanisms, their pathways are highly integrated. Endocrine hormones are the work unit of the endocrine mechanisms, and are chemical messengers synthesized in specialized cells and then released into the circulation where they are available for uptake by and action on remote tissues. The fundamental concept is that hormones are chemical mediators of metabolism.
Endocrine hormones are those that are released into the circulation for action in very remote areas (such as India or China). In contrast, autocrine hormones are those that act on the cell that secreted them. Paracrine hormones are the pleasant middle brother, since they act on neighbour cells.
Classes of hormones
There are three large classes of hormones that act in the endocrine system: peptide hormones, steroid hormones and amino acid derivatives.
Peptide and polypeptide hormones are small monomer strings of amino acids. While some peptide hormones can be relatively small, others can be very large and complex, with both primary and secondary structures. Peptide hormones are generally transcribed within the nucleus to mRNA, where they are often first translated into pro-hormones. Necessary modifications can be made in the golgi apparatus, where hormones are also packaged for secretion. Secretion then awaits a suitable stimulus.
Steroid hormones are derived from the metabolism of cholesterol through a series of enzymatic steps. I guess they are sent out in the same way as peptide hormones, eh?
Amino acid derivatives are derived from enzymatic modifications of an amino acid. Thyroid hormones triiodothyronine (T3) and thyroxine (T4) fall into this class, and are produced from the biological iodination of tyrosine residues in thyroglobulin.