The root cause of acromegaly is excessive secretion of growth hormone (GH), caused by either a pituitary tumor or a non-pituitary tumor. Too much GH leads to a cascade of events and the symptoms of acromegaly.
Main Acromegaly Cause: Pituitary Tumor
Benign pituitary tumors, also known as pituitary adenomas, are split into 2 categories, depending on how big they are. There are micro-adenomas and macro-adenomas. Micro-adenomas are less than 1 cm big; macro-adenomas are bigger than 1 cm. Keep in mind that the pituitary gland is about 1 cm big, so if the tumor has grown to at least the same size as the gland, it should be considered "big."
Most pituitary tumors that over-secrete growth hormone are macro-adenomas.
Regardless of their size, all pituitary tumors start the same way: a pituitary cell spontaneously grows and multiplies when it shouldn't. That's caused by a genetic mutation in the pituitary cell—a mutation that is sometimes inherited from your parents, but in most people the mutation is new and not present at birth. The gene mutates all of a sudden, and medical researchers are working hard at understanding why.
Pituitary cells continue to multiply, and the tumor grows. The growth rate varies among people with acromegaly: For some, the tumor develops slowly and, therefore, symptoms develop slowly. For others, the tumor grows quickly.
Here's another difference in how pituitary tumors develop in people with acromegaly: The location of the tumor influences what symptoms you have. The tumor can start to press on another section of the pituitary gland, causing that section to over- or under-secrete hormones. For example, the tumor can press on the part of the pituitary that controls thyroid hormones, leading to a thyroid disorder.
Because pituitary adenomas can cause changes with other pituitary hormones, your doctor should check all of your hormone levels if you have acromegaly.
Much Rarer Acromegaly Cause: Non-pituitary Tumors
It is possible to have a tumor in another part of your body that causes too much growth hormone and acromegaly. These tumors are usually in other parts of your brain, the pancreas, or the lungs.
The tumor can produce growth hormone, even though it isn't part of the pituitary gland. More often, the tumor produces growth hormone-releasing hormone (GHRH), the hormone that tells the pituitary gland to make more GH. If the non-pituitary tumor is secreting GHRH, then your pituitary gland will respond to it by making more GH than your body needs, which leads to acromegaly.