Final answer:
A tertiary endocrine disorder can be caused by a pituitary tumor, overactivity of the non-trophic gland, or an overactive hypothalamus.
Step-by-step explanation:
A tertiary endocrine disorder can be caused by a pituitary tumor (option a), where the tumor affects the function of the pituitary gland and disrupts hormone production.
Another possible cause is overactivity of the non-trophic gland (option b), where the gland produces excessive amounts of hormone. Additionally, an overactive hypothalamus (option c) can also lead to a tertiary endocrine disorder.