Hi Indigogal
According to Laura Norman's Reflexology Handbook, you'd work direct; Solar plex/diaphragm, chest/lung, shoulder/arm, neck, heart, thyroid and helper, parathryoid all toes with emphasis on pituitary, brain, pancreas, adrenals,; indirect; chest/lung, thymus, shoulder/arm neck/throat.
hope this helps. She doesn't give her reasons and I'm not clever enough to give you definite reasons why you'd work the points. She covers all the endocrine system so I'd say she's taking the approach that you'd be looking to balance out hormone levels etc.
It's a start anyway.
Be good
Setanta