You can discard “Thunder Dragon” to activate its effect even if it would not go to your Graveyard. For example, when “Banisher of the Light” is face-up on the field.
If you have your opponent’s “Thunder Dragon” in your hand because of “Exchange”, you can discard it to activate its effect and search your Deck for up to 2 “Thunder Dragons”.
You can discard “Thunder Dragon” to activate its effect even if you have 2 “Thunder Dragon” already in your Graveyard.
Netrep Rulings:
“Thunder Dragon’s” effect is an activated effect.
“Thunder Dragon” may be discarded to search for 0 “Thunder Dragons” in the player’s deck.
The effect of “Thunder Dragon” may not be activated if it is discarded through another effect.