Final answer:
The first diagnostic option for a suspected endocrine disorder is typically an immunoassay of blood hormone levels, which helps to evaluate the function of endocrine organs.
Step-by-step explanation:
In a patient suspected of having an endocrine disorder, the first diagnostic option that may be considered is A. Immunoassay of blood hormone levels.
Endocrinologists use a variety of laboratory tests to assess and diagnose endocrine disorders, and blood tests are central to this process. By measuring hormone levels in the blood following stimulation or suppression tests, such as the fasting glucose test or the A1C test for diagnosing diabetes mellitus, a more accurate assessment of endocrine organ function can be made.
CT scans, MRIs, and antibody testing might be used later in the diagnostic process or for different indications but are not typically the first line of investigation for endocrine disorders.