To have a defect in $only$ $one$ specific endocrine pathway, only the final gland for that pathway could be affected.
Work Step by Step
A defect higher up, such as at the hypothalamus or pituitary gland, would cause defects in multiple pathways. For the problem to be limited to only one, the defect would have to be present in a component that affects only one individual pathway. That necessarily must be the last one, as any higher in the process would affect other pathways as well.