The shoest hormone is:

Thyrotropin-releasing hormone is one of the smallest hormones in the body, consisting of a miniature chain of just three amino acid building blocks. It is made by a cluster of nerve cells in the hypothalamus, an area at the base of the brain just above the pituitary gland. This nerve cell cluster is known as the paraventricular nucleus. Longest hormone is HCG (250 amino acids)