I'm 70 year old man who thought he was in good health until SVT

@jfcbuzz1980 I wonder if issues with your parathyroid are causing the SVT (with or without surgery). This is a stretch but I took a parathyroid hormone analog for bones and it causes fast heart rate while in the system. Just a thought.

Are you on any meds for the SVT? Did you wear a monitor or have a smartwatch or Kardia.

So sorry you are going through this. I hope your cardiologist is helpful. I saw an electrophysiologist (afib) and they were the most helpful.

A tumor on your pituitary may play with the levels of thyroxin in your thyroid. If it has caused elevated levels of thyroxin, you can get elevated heart rate. I'm sure, though, that your specialists have thought of that, and they have, or are about to, rule it out....but perhaps not.
Atrial fibrillation is included in the 'cloud' of SVT. Unless you can be monitored and a recording taken during a bout of AF, it won't be a consideration. For that, a Holter Monitor or a Loop Recorder (implanted) would need to monitor your heart's rhythm and rate for as much as an entire month, but usually just a day or two suffices.

Was a rate ever determined? A rate higher than about 140 can cause light-headedness and breathlessness, including syncope (loss of blood pressure to the point of fainting). A cardiologist who sees this high a rate might want you on a Direct Oral Anti-Coagulant, or DOAC. The problem is that blood not being moved along smartly by the heart, and a heart in arrhythmia often does not move the blood efficiently (and that accounts for the syncope), will promote clotting to commence. A clot that gets shoved out of a heart and into its own arteries, or into the lungs, or into the brain......not good at all.

I don't know what else to tell you except that you may want to ask about a Holter monitor and from there get medication to control rate, to control arrhythmia, and definitely to control clotting. I am on Eliquis, a drug called apixaban, but there is also Xarelto (rivaroxaban). They are not thinners, they're anti-clotting agents which only retard the process, they don't prevent it entirely. You'll still clot, but minutes later, and that gives a wonky heart enough time to get more blood moved out of itself before it begins to clot.

There are many types of pituitary tumors and they can affect different hormones. So you would not direct a straight line to thyroid stimulating hormone without at least common thyroid hormone tests. And also a CT/MRI scan would reveal where the tumor is on the gland and you would get a better idea of the kind of tumor you have. So you have said you have had every test known to man. Did they tell you anything about the tumor and what if anything they want to do about it? Your tumor and SVTs many very well be 2 very distinct pathologies and have nothing to do with each other.
How often do you have SVTs and how high does you heart rate go? Have you seen an electrophysiologist (EP) cardiologist for a proper diagnosis? That they found the tumor was maybe a result of scans. Did the MD(s) involved tell you what if anything can be done for the SVTs. There are a number of treatments involved if you are having enough episodes. You leave out a lot of information on the presentation of the SVTs. BTW using a Holter monitor is commonly used to help document SVTs.
Here is what Mayo Clinic says about SVTs.
https://www.mayoclinic.org/diseases-conditions/supraventricular-tachycardia/diagnosis-treatment/drc-20355249

Since the pituitary gland secretes several hormones, and a tumor usually produces excess hormone, it’s very important that a pituitary hormone panel be done on blood to measure these levels. There are 2 in particular that could be related to the SVT- ACTH stimates the adrenal glands to produce cortisol and epinephrine (adrenaline). High levels can cause SVT. TSH stimulates the thyroid gland to produce thyroxin and high levels cause SVT. These are blood tests that your PCP, cardiologist or an endocrine specialist can check. Also, please research or ask your cardiologist about 3 mechanisms that may stop the SVT: Val Salva, ice water face plunge, and carotid massage. Don’t do carotid massage without permission from Dr. he may want carotid ultrasound first . Also a medication like a beta blocker may prevent the episodes until cause determined. I hope you get this resolved soon!

Reply to comment to 70 yr old man who thought he was in good health until SVT.
—— I have SVT and have written here several times about living with it for many years just fine. (You can search.)
But I do not believe you have such a benign case as mine since yours just began at 70 along with other coincidental problems. The above replies seem very thorough and good advice.