A B737 CAPT DECLARES A MEDICAL EMER BECAUSE OF HIS PHYSIOLOGICAL NEEDS AND IS GIVEN PRIORITY TO THE ARPT. THE FAA THREATENS A VIOLATION.

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I WAS AT THE CTLS DSNDING THROUGH 9500 FT ON ZZZ APCH AIRSPACE WHEN I HAD AN IMMEDIATE NEED TO GO TO THE BATHROOM. I INFORMED THE FO AND HE LOOKED SURPRISED. I TOLD HIM I WAS THINKING ABOUT DECLARING A MEDICAL EMER AND HE LOOKED MORE SURPRISED SINCE THIS WAS SUCH A SUDDEN ONSET. MY CONDITION IMMEDIATELY WORSENED AND I RADIOED ATC THAT I WAS DECLARING A MEDICAL EMER FOR A PAX; BUT WOULD NOT NEED FURTHER ASSISTANCE ON THE GND. I SAID 'PAX' PARTLY BECAUSE I WAS EMBARRASSED AND PARTLY BECAUSE I WAS DISTR BY HOW I WAS FEELING. ATC SAID OK AND TURNED US TOWARDS THE FINAL. THE FO STARTED TO INPUT THE FMC FOR THE NEW CLRNC. I CLICKED OFF THE AUTOPLT AND STARTED THE R TURN WHILE IN THE DSCNT. I CLICKED OFF THE AUTOPLT WHILE IN A 1500 FPM DSCNT AND REALIZED THAT I HAD DSNDED THROUGH MY ASSIGNED ALT OF 8000 FT. I STARTED MY CLB AS SOON AS I RECOGNIZED MY ERROR AND AT THE SAME TIME ATC SAID TO MAINTAIN 8000 FT. THE LOWEST ALT THAT I REMEMBERED WAS JUST UNDER 7400 FT. I PROMPTLY LEVELED OFF AT 8000 FT AND LANDED NORMALLY. ALTHOUGH THE FO IS THE MONITORING PLT HE HAD LITTLE TIME TO REACT. FROM THE MOMENT I RECEIVED THE CLRNC AND STARTED TO TURN AND DISCONNECTED THE AUTOPLT; THE INCIDENT OCCURRED VERY QUICKLY. THIS EVENT WAS EVERYTHING TO DO WITH MY ACTIONS AND SHOULD IN NO WAY IMPACT HIS GOOD NAME. SOMETIMES IT IS EASY TO FORGET HOW QUICKLY THINGS CAN GO WRONG. AFTER PARKING I LEFT FOR THE BATHROOM AND EMS ARRIVED TO THE ACFT. THE FO EXPLAINED TO THEM IT WAS THE CAPT WHO HAD TO GO. I ALSO HAD A CONVERSATION WITH THE TWR SUPVR WHO INITIALLY WAS UPSET THAT THERE WAS NOT A PAX EMER BUT WHEN I EXPLAINED THAT IT WAS ME AND HE SAID THAT HE DID NOT HAVE A PROB WITH THE INCIDENT. I SPOKE WITH THE TRACON SUPVR AND I ADMITTED THAT I HAD MADE A MISTAKE AFTER DECLARING A MEDICAL EMER. HE SAID ALTHOUGH HE WAS SYMPATHETIC HE WAS REQUIRED TO FILE A PLTDEV. IN RETROSPECT I SHOULD HAVE GIVEN CTL OF THE ACFT OVER TO THE FO IMMEDIATELY; HOWEVER WHEN THE THOUGHT CAME TO MIND I THOUGHT THAT I WOULD BE ABLE TO GET ON THE GND FASTER AND KEEP MY MIND ON SOMETHING ELSE OTHER THAN HOW I FELT. IN MY 24 YRS OF FLYING FOR THIS COMPANY I HAVE NEVER FELT THE NEED TO GO SO STRONGLY THAT CLOSE TO THE GND OR HAD ANY OTHER MEDICAL ISSUES. THE SECOND MISTAKE WAS SAYING PAX MEDICAL EMER AND NOT ADMITTING IT WAS ME. HONESTY IS ALWAYS THE BEST POLICY REGARDLESS OF THE EMBARRASSMENT. HAVING THIS EXPERIENCE HAS ALLOWED ME THE OPPORTUNITY TO SELF REFLECT. IN THE FUTURE SHOULD ANYTHING MEDICALLY HAPPEN TO ME I WILL IMMEDIATELY HAND OVER THE CTLS TO THE FO. SUPPLEMENTAL INFO FROM ACN 806427: WHILE DSNDING INTO ZZZ GOING THROUGH APPROX 9500 FT THE CAPT SAID HE HAD TO GO TO THE BATHROOM IMMEDIATELY AND WAS GOING TO DECLARE AN EMER. I THOUGHT HE WAS KIDDING UNTIL HE TOLD ATC THAT WE HAD A MEDICAL EMER. ATC GAVE US A SLIGHT R TURN AND TOLD US TO EXPECT RWY XL INSTEAD OF RWY Y. THE CAPT INSTRUCTED ME TO BUILD THE APCH IN THE FMC AND TUNE THE NAV RADIOS FOR A RWY XL ARR. DURING THE SHORT TIME I WAS PROGRAMMING THE FMC HEAD DOWN; THE CAPT DISENGAGED THE AUTOPLT AND I WAS UNAWARE THAT WE DSNDED BELOW OUR ASSIGNED ALT OF 8000 FT. WHEN I LOOKED UP WE WERE CLBING THROUGH 7500 FT TO REACQUIRE OUR ORIGINALLY ASSIGNED ALT OF 8000 FT. ALTHOUGH THIS HAPPENED VERY QUICKLY I SHOULD HAVE NOT PROGRAMMED THE FMC AT THAT CRITICAL PHASE OF FLT. I BELIEVE THAT NEXT TIME I WILL BE MORE AWARE ESPECIALLY WHEN THE AUTOPLT IS TURNED OFF AND SUGGEST THAT WE JUST TUNE THE NAV RADIOS AND NOT GO HEADS DOWN TO PROGRAM THE BOX FOR A VISUAL WHEN AT A FAMILIAR ARPT.