An endocrine gland may be secreting too little hormone because the gland cannot function normally. This is termed primary hyposecretion. Examples of primary hyposecretion include (1) destruction of the adrenal cortex, leading to decreased cortisol secretion, and (2) dietary deficiency of iodine leading to decreased secretion of thyroid hormones. There are many other causes— infections, toxic chemicals, and so on—all having the common denominator of damaging the endocrine gland.
In contrast to primary hyposecretion, a gland may be secreting too little hormone because there is not enough of its tropic hormone. This is termed secondary hyposecretion. There may also occur (though rarely) a situation called tertiary hyposecretion. In such a case, there is a deficiency of hormone production from a gland and also a deficiency of the gland’s tropic hormone. However, in tertiary hyposecretion the tropic hormone–producing gland is functional; instead, a third hormone (from the hypothalamus) that normally "starts the ball rolling" by stimulating the secretion of the tropic hormone is missing. Thus, hyposecretion can occur at any of three levels in situations where the secretion of a hormone is linked to the secretion of two other hormones in turn.