Causes of reduced bioavailability of a drug includes
High first pass metabolism
Increased absorption
IV drug administration
High lipid solubility
When the metabolism of a drug is high during its passage from the site of absorption into the systemic circulation, the rate, and extent of absorption of that drug decreases. absorption- amount of drug reaching the systemic circulation from the site of administration (Ref.Essentials of medical pharmacology TD Tripathi 7th edition page no.16,27)
Endocrinology