Proteinaceous substances are those that are made up of amino acids and have a complex three-dimensional structure. They are typically involved in various biological functions and processes. Among the given options, the only proteinaceous substance is FSH (follicle-stimulating hormone), which is a hormone produced by the pituitary gland.

- FSH (Follicle-Stimulating Hormone):
FSH is a proteinaceous hormone that plays a crucial role in the reproductive system. It is responsible for the growth and development of ovarian follicles in females and the production of sperm in males. FSH is composed of two subunits, alpha and beta, and the beta subunit contains the active site responsible for its biological activity.

- Thyroxine:
Thyroxine, also known as T4, is a hormone produced by the thyroid gland. It is involved in the regulation of metabolism and growth. However, thyroxine is not proteinaceous in nature. It is a derivative of the amino acid tyrosine but does not have a complex three-dimensional protein structure.

- Progesterone:
Progesterone is a steroid hormone involved in the menstrual cycle and pregnancy. It is primarily produced by the corpus luteum in the ovaries and the placenta during pregnancy. Progesterone is not proteinaceous in nature but belongs to the class of steroids, which are derived from cholesterol.

- Oxytocin:
Oxytocin is a hormone produced by the hypothalamus and released by the posterior pituitary gland. It plays a role in various physiological processes, including childbirth, lactation, and social bonding. Although oxytocin is involved in reproductive functions, it is not proteinaceous but rather a peptide hormone composed of nine amino acids.

In summary, among the given options, FSH is the only proteinaceous substance. Thyroxine, progesterone, and oxytocin are hormones but not proteinaceous in nature.