Unless I'm missing something, this does not appear to be correct. Thunderlance (one word) does not add INT bonus to To-Hit or Damage rolls, it allows you to add caster level to STR.
EDIT:The thunderlance strikes with a Strength score equal to 12 + your caster level (maximum +15) which replaces your own Strength score when you make attack and damage rolls with the spell.
Meh, the spell was printed in two places with different text making it operate fairly differently. Brilliant! The above quote is from Forgotten Realms. The one below which does give INT -> STR or CHA -> STR is from the Spell Compendium.
Instead of using your Strength modifier, you use the higher of your Intelligence modifier or Charisma modifier as a bonus on attack rolls and damage rolls.