If the pituitary problem slowed your thyroid in the past, you were probably gaining weight, which in itself is a strain on the heart, especially when exercising.  The good thing would be that with the thyroid treatment that may gradually become less of a problem.

The heart will race for many reasons, including exercising faster than fits the condition of the heart or its arteries, or lung damage, or poor circulation in the body.  Excess adrenaline will make it race, but that is not likely here I think. The heart knows it needs to work harder, perhaps because there is an oxygen level sensor, the carotid body in the carotid arteries which signals the brain which then signals the heart.

There is a lot of readable information on the pituitary at http://www.pituitary.org/library/about.aspx

You mentioned hypopituitary - has he checked your thyroid? Hypothyroidism is characterized by brain fog. It can certainly cause ADHD-like symptoms.

Another consideration as a cause of racing heart is Dysautonomia, associated with inappropriate epinephrine release. It certainly could cause all sorts of symptoms, but not likely ADD-like ones. Your dose of Synthroid could be too high. Stimulant use can also affect your heart rate and response to exercise.Endocrine malfunctions can certainly create brain-fog with attentional and other malfunctions, as can fibromyalgia, depression, anxiety disorders, early dementing illness, disorders causing delerium (like electrolyte abnormalities, renal or liver failure), sedating medication use, recreational drug use, and especially Sleep Apnea or other causes of inadequately restful sleep. A former partner seemed to diagnose everyone with ADD, especially his staff, but unfortunately it was to find extra amphetamine doses when his tolerance exceeded his own supply - and he DIDN’T have ADD. A lot of his unresponsive patients had Sleep Apnea, hypothyroidism, or something he’d overlooked to the patient’s detriment. And a person WITH ADD who doesn’t treat the above listed problems often does not respond to treatment. IT IS CERTAINLY POSSIBLE TO HAVE ADD AND MEDICAL PROBLEMS SIMULTANEOUSLY, so beware a doctor who STILL DOESN’T BELIEVE IN ADD. After finding the SECOND PROBLEM, they take you off treatment and you don’t do well. If you do well on no meds for ADD, you probably don’t have ADD.