- Hormone that acts on the target tissue and causes change in activity
- Example:- Triiodothyronine (T3) and Thyroxine (T4) are direct hormones because they act directly on tissues throughout the body to increase metabolism, heart rate, temperature, etc.
- Thyroid Stimulating Hormone (TSH) is a tropic hormone because it acts on another endocrine gland (the thyroid) to stimulate the release of T3 and T4, rather than acting on body tissues itself.
See HPT Axis for more information on the thyroid axis