Professional and Linguistic Assessments Board (PLAB) Practice Exam

What is the first test in screening for prolactinoma?

1. TSH > Prolactin

2. Prolactin > TSH

3. Corticosteroids

4. ACTH levels

The correct answer is: TSH > Prolactin

In the context of screening for prolactinoma, the correct approach is to first measure thyroid-stimulating hormone (TSH) levels before checking prolactin levels. This sequence is critical because elevated prolactin can be secondary to hypothyroidism. In cases where the TSH level is abnormal, indicating thyroid dysfunction, it could lead to secondary hyperprolactinemia, which is not indicative of a prolactinoma. Thus, determining TSH levels first helps to rule out any thyroid-related causes of elevated prolactin. If TSH levels are normal, then elevated prolactin can be analyzed further, leading to the consideration of prolactinoma as a potential diagnosis. Measuring corticosteroids or ACTH levels is generally not part of the initial diagnostic approach for prolactinoma. These are more specific tests used in the evaluation of other endocrine disorders, such as Cushing's syndrome or adrenal insufficiency, rather than in the context of prolactin abnormalities. This order of assessment ensures a more accurate diagnosis by differentiating between primary and secondary causes of elevated prolactin levels.