In which of the following form is Anti Diuretic Hormone (ADH) circulated in the plasma?

ADH is produced by the magnocellular neurons arising bilaterally in the supraoptic nuclei of hypothalamus. These neurons project through the pituitary stalk to posterior lobe of pituitary. From there in response to appropriate stimulus, ADH in the free form is released into the circulation and is carried with serum protein globulin to the target organs. Ref: Sections 1 - 10 and Index, Volume 1 By Joan Rodes , Page 680 ; Essential of Endocrinology and Reproductive Physiology, Page 32.