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EXPLANATION
✔Correct answer:
A serum somatomedin C (insulin-like growth factor) level exceeding 300 ng/mL. Acromegaly is a disorder characterized by excessive growth hormone (GH) secretion that occurs in adulthood, after the epiphyseal growth plates have closed. This excessive GH production leads to an increase in insulin-like growth factor 1 (IGF-1), also known as somatomedin C, which mediates many of the effects of growth hormone, including cell growth and replication. In the context of acromegaly, elevated serum levels of somatomedin C (IGF-1) are a hallmark of the disease, reflecting the overproduction of growth hormone by the pituitary gland. The measurement of IGF-1 levels is crucial for the diagnosis of acromegaly because IGF-1 levels are stable throughout the day and are less variable than GH levels, providing a reliable indicator of GH excess.
Imagine your body is a garden, and growth hormone is the water supply while somatomedin C (IGF-1) is the growth of the plants. In a normal setting, watering the garden promotes healthy plant growth. However, if you were to excessively water the garden (akin to the overproduction of GH in acromegaly), you'd notice an overgrowth of plants (elevated IGF-1 levels), indicating that something is amiss with the water supply.
Acromegaly is primarily caused by a pituitary adenoma, which is a benign tumor in the pituitary gland that secretes excessive amounts of growth hormone (GH). GH has direct effects on tissues, but its most significant effects are mediated by IGF-1, which is mainly produced in the liver in response to GH stimulation. IGF-1, in turn, promotes growth and metabolic effects throughout the body, including increased protein synthesis, cartilage formation, and organ growth. In adults, excess GH and IGF-1 lead to acromegaly, characterized by bone growth in the face, hands, and feet, and organ enlargement. This condition also affects glucose metabolism, leading to insulin resistance.
✘Incorrect answer options:
A plasma glucose level lower than 70 mg/dL. This option describes hypoglycemia, which is not directly associated with acromegaly. In fact, acromegaly can lead to insulin resistance and, consequently, higher glucose levels due to the counter-regulatory effects of growth hormone on insulin action.
Decreased growth hormone levels after an oral glucose challenge test. In a healthy individual, an oral glucose tolerance test (OGTT) would lead to a suppression of GH levels. However, in patients with acromegaly, GH levels fail to suppress following an OGTT, which is a diagnostic criterion for the condition. Therefore, expecting decreased GH levels after an OGTT in a patient suspected of acromegaly would be incorrect.
Elevated levels of insulin in the blood. While patients with acromegaly may exhibit insulin resistance due to the antagonistic effects of GH on insulin, leading to elevated insulin levels as the body tries to compensate, this is not a direct diagnostic criterion for acromegaly. The key diagnostic tests involve measuring GH and IGF-1 levels, and the response of GH to an oral glucose challenge.
References
- Melmed, S., Polonsky, K. S., Larsen, P. R., & Kronenberg, H. M. (2020). Williams Textbook of Endocrinology. Elsevier Health Sciences. This text provides comprehensive coverage of endocrine diseases, including acromegaly, detailing diagnostic criteria and management approaches.
- Katznelson, L., Laws, E. R. Jr., Melmed, S., et al. (2014). Acromegaly: An Endocrine Society Clinical Practice Guideline. The Journal of Clinical Endocrinology & Metabolism, 99(11), 3933–3951. This guideline outlines the recommended practices for the diagnosis and management of acromegaly, including the use of IGF-1 levels and GH suppression tests for diagnosis.