$(a)$ Define hormone. Write four characteristics of hormones in humans.
$(b)$ Name the disorder caused by the following situations:
$(i)$ Under secretion of growth hormone
$(ii)$ Over secretion of growth hormone
$(iii)$ Under secretion of insulin
$(iv)$ Deficiency of iodine

(A-D) Hormones: Hormones are chemical messengers that regulate biological processes in living organisms.
Characteristics of Hormones:
$1$. They are produced by ductless glands,meaning their secretion is released directly into the blood.
$2$. They are released in trace amounts,i.e.,in very small quantities.
$3$. They act on specific tissues or organs known as target organs.
$4$. They are generally slow in action.
$5$. They act away from the site of production.
$(b)$ $(i)$ Dwarfism
$(ii)$ Gigantism
$(iii)$ Diabetes mellitus
$(iv)$ Goitre