It's really no different from other hormones: they are only synthesized in one organ and then released into the body. Sex hormones are mainly (not exclusively) synthesized in the gonads, etc.

It is fascinating that the body regulates thyroid hormones the same way a chemical plant would regulate a key chemical today. The pituitary gland monitors the level of thyroxine in the blood and emits thyroid stimulating hormone in greater or lesser amounts if the thyroxine level is too low or too high. The thyroid, physically located separate from the pituitary, then produces thyroxine depending on the TSH level it senses. If the sensing function and the synthesis function were physically located at the same place, the body couldn't control what was happening.