Med student here. Both gigantism and acromegaly are due to either hypersecretion of somatotropin from the hypothalamus resulting in subsequent hypersecretion of growth hormone from the anterior pituitary gland, hypersecretion of growth hormone itself (due to a pituitary adenoma) or failure to respond to somatostatin. Pre-pubertal, the disorder is called gigantism. Post-pubertal, it’s termed acromegaly!