Which of the following hormones uses calcium ions as a second messenger to transduce its signal within the cell?
Cortisol
Adrenaline
FSH
Calcitonin
The hormone that uses calcium ions as a second messenger to transduce its signal within the cell is Calcitonin. Cortisol is a steroid hormone that acts directly on intracellular receptors, while adrenaline and FSH use cyclic AMP (cAMP) as a second messenger. Therefore, option 4 is the correct answer.