Acromegaly is a rare and underdiagnosed disorder caused, in more than 95% of cases, by a growth hormone (GH)-secreting pituitary adenoma [1], [2], [3]. Pituitary carcinomas are very rare and less than 200 cases have been reported [4]. The remaining cases of acromegaly are mainly due to growth hormone releasing hormone (GHRH)- hypersecretion from a hypothalamic tumour or from an ectopic GHRH-secreting tumour [5]. In the literature there are around 130 cases of acromegaly caused by GHRH-secretion by neurondocrine tumours, mostly of lung and pancreatic origin [6]. Ectopic GH hypersecretion is exceedling rare with only two cases reported so far: an intramesenteric pancreatic islet-cell tumour [7] and a non-Hodgkin´s lymphoma [8]. Acromegaly affects males and females equally and the average age at diagnosis ranges from 40 to 50 years [9], *[10].

The GH hypersecretion leads to overproduction of insulin-like growth factor 1 (IGF-1) mostly by the liver which results in a multisystem disease characterized by acral overgrowth, facial disfigurement, hyperhidrosis, and several comorbidities (e.g., cardiovascular, metabolic, osteoarticular, respiratory, and neoplastic diseases). If not early diagnosed and properly treated, it impairs quality of life and increases mortality [9], *[10].

High levels of IGF-1 are responsible for most of the clinical manifestations of acromegaly [9], *[10]. IGF-1 levels are also the key factor in the diagnosis and monitoring of acromegaly, but GH measurements should not be ignored, as these hormones reflect different aspects of the disease [11]. Indeed, GH is usually, but not always, a measure of the secretory activity of the tumor and provides a correlate of pituitary GH secretion. By contrast, IGF-1 is a measure of biochemical and biological activity of the disease and the peripheral response to circulating GH *[11], [12].

More than 130 years after the French neurologist Pierre Marie coined the term “Acromegalie” in late 1885 [13] the diagnosis of acromegaly is still challenging [14]. In most patients with acromegaly both GH and IGF-1 levels are elevated but discordant results are not rare [15], *[16]. A meta-analysis of 39 studies totaling 7071 patients has shown that the pooled discordance rate between GH and IGF-I was 25.7% (95% CI: 22.3–29.4), and the predominant format was that of elevated IGF-I with normal GH levels (15.3%, 95% CI: 12.5–18.7). The use of ultrasensitive GH assays resulted in higher discordance rates (30.7%, p = 0.04). [15].

“Micromegaly” describes a subgroup of patients with clinically evident acromegaly and high IGF-1 concentrations associated to apparently normal basal GH (bGH) and often to a GH nadir < 1 ng/mL after a glucose load during the oral glucose tolerance test (OGTT) *[11], *[17]. The term was initially coined by Barkan and coworkers who, in 2002, reported 16 patients with those features [17]. It is still controversial whether “micromegaly” is a distinct clinical entity or a classic acromegaly in early stages *[11], *[17], *[18]. This review article mainly aims to evaluate this subject, especially based on the characteristics of patients reported in the literature. To properly diagnose "micromegaly", it is important to be aware of the various factors that can alter serum GH and IGF-1 levels, causing discordant results.