"Does not name a type" error with Serial.print

I am getting an error that I have never seen before and don't know how to fix. Basically I am trying to print a string (stored in thermistor) to the serial monitor. When I run the code I get an error message that says: exit status 1 'Serial' does not name a type.

What can I do fix this error?

void setup() {
Serial.begin(9600);
pinMode(4, OUTPUT);
pinMode(6, OUTPUT);
pinMode(8, OUTPUT);

}

void loop() {

} <==== ERROR: loop() ENDS TOO EARLY
String heartMonitor = codeWrite(4, HIGH, LOW, HIGH);
String lilyPad = codeWrite(6, HIGH, LOW, LOW);
String thermistor = codeWrite(8, HIGH, HIGH, LOW);
Serial.println(thermistor);
delay(1000);


String codeWrite(int pin, char output1, char output2, char output3)
{
  digitalWrite(pin, output1);
  int A = digitalRead(pin);
  delay(1);
  digitalWrite(pin, output2);
  int B = digitalRead(pin);
  delay(1);
  digitalWrite(pin, output3); 
  int C = digitalRead(pin);
  delay(11);
  String code = String(A) + String(B) + String(C);
  <==== ERROR: MISSING RETURN STATEMENT
}