Which hormone is known to exhibit diurnal variation?

Cortisol is well known for its diurnal variation, which means that its levels fluctuate throughout the day, typically peaking in the early morning upon waking and gradually declining throughout the day, reaching their lowest levels late in the evening. This pattern is influenced by the body’s internal clock, also known as the circadian rhythm, and is integral to various physiological processes, including metabolism, immune response, and stress management.

Understanding cortisol's diurnal rhythm is essential in clinical chemistry, as it affects diagnostic interpretations. For example, abnormal cortisol levels at specific times may indicate conditions like Cushing's syndrome or adrenal insufficiency. Therefore, when measuring cortisol for diagnostic purposes, the timing of specimen collection is critical to achieve accurate assessment.

In contrast to cortisol, other hormones like adrenaline, insulin, and thyroxine do not exhibit the same pronounced diurnal variation. Adrenaline levels can fluctuate based on stress and physical activity rather than a consistent daily pattern. Insulin secretion is more closely tied to dietary intake and blood glucose levels, rather than a strict daily rhythm. Similarly, while thyroxine levels can display some variation, they are not characterized by a clear diurnal cycle like cortisol. This distinction highlights why cortisol is the hormone best known for exhibiting di