Hangman game performance
For the first function you could use set operations and the built-in string module:

import string

ALPHABET = set(string.ascii_lowercase)

def get_available_letters(letters_guessed):
    '''
    letters_guessed: iterable, what letters have been guessed so far
    returns: string, comprised of letters that represents what letters have not
      yet been guessed.
    '''
    return "".join(sorted(ALPHABET - set(letters_guessed)))


Note that I renamed your function/variables to adhere to Python's official style-guide, PEP8.

If you don't care that the return value is sorted, or a string, you can just return the set right away:

def get_available_letters(letters_guessed):
    '''
    letters_guessed: iterable, what letters have been guessed so far
    returns: set, letters that have not yet been guessed.
    '''
    return ALPHABET - set(letters_guessed)


In your actual game function, here are a few nitpicks:


Use str.format for an easier way to put some value somewhere in a text. This changes print ('I am thinking of a word that is ') + intro + (' letters
long.') to print 'I am thinking of a word that is {} letters
long.'.format(len(secretWord)) and print ('You have ') + str(mistakesMade) + (' guesses left.') to print ('You have {} guesses left.'.format(mistakesMade))
while mistakesMade &gt; 0 and mistakesMade &lt;= 8 and wordGuessed is False: could be while 0 &lt; mistakesMade &lt;= 8 and not wordGuessed
Use a set also for lettersGuessed, then if guess in lettersGuessed: becomes \$\mathcal{O}(1)\$, instead of \$\mathcal{O}(n)\$
if wordGuessed == True: can just be if wordGuessed: