Final answer:
Cortisol is the substance that exhibits diurnal variation, with levels being higher in the morning and decreasing throughout the day. This variation can impact medical diagnoses related to abnormally high or low levels of cortisol.
Step-by-step explanation:
Of the substances listed, cortisol shows a diurnal variation, which means its levels in the blood change throughout the day. Cortisol levels are typically higher in the morning upon waking and decrease throughout the day. This pattern is linked to the body's circadian rhythm. Diurnal variation is not seen with troponin, used to diagnose heart attacks, or potassium and ammonia, which are more stable electrolytes and waste products, respectively.
Diurnal variation is an important concept in medicine, as timing of sample collection can affect diagnosis and management of diseases. For instance, measuring cortisol at the wrong time of the day may lead to abnormally high levels of cortisol or abnormally low levels of cortisol, which can suggest conditions such as Cushing's syndrome or Addison's disease, respectively.
Hormones like aldosterone also contribute to the body's fluid and electrolyte balance, but they do not show the same type of diurnal variation that cortisol does. Instead, their levels may change in response to factors like blood pressure, blood volume, and the body's sodium and potassium levels.