Understanding the Tests: MRI and Hormone Evaluations

Confirming a diagnosis of Pituitary Stalk Interruption Syndrome (PSIS) is a two-part puzzle. First, doctors use imaging to look at the physical structures of the brain. Second, they use specialized blood tests to see how those structures are actually performing. While the MRI provides the “map,” the hormone tests tell us if the “bridge” is carrying the necessary traffic ^[1][2].

The MRI: Mapping the Anatomy

As discussed in previous pages, the MRI is the primary tool for identifying the physical signs of PSIS. Your child’s radiology report will likely mention several key areas:

• The Pituitary Stalk: Doctors look to see if the stalk is present, thin (hypoplastic), or completely absent ^[3].
• The Anterior Pituitary: They measure the size of the front lobe of the gland to see if it is smaller than expected (hypoplastic) ^[1].
• The Ectopic “Bright Spot”: This is the posterior pituitary. In PSIS, it is often found in the wrong place, usually higher up in the brain ^[1].
• Other Midline Structures: Because the pituitary develops alongside other parts of the brain, doctors also check the optic nerves (for vision), the corpus callosum (which connects the two sides of the brain), and the septum pellucidum ^[3][4]. Associated findings in these areas may prompt referrals to ophthalmology or developmental specialists.

Important Note: An ectopic “bright spot” does not automatically mean your child will have trouble with water balance (Vasopressin deficiency). The ectopic location does not by itself diagnose the deficiency; function is assessed from thirst, urine output, sodium/osmolality, and specialist evaluation ^[5][6]. Additionally, MRI anatomy alone cannot perfectly predict exactly which anterior hormones will be deficient.

The Blood Tests: Measuring the “Axes”

The pituitary doesn’t just release one hormone; it controls several different “axes” or pathways. To see if these pathways are working, doctors perform a hormone panel.

Why Simple Blood Draws Aren’t Enough

For some hormones, a single blood draw in the morning is sufficient. For example, Free T4 (thyroid) is usually steady enough to measure once ^[2]. However, hormones like Growth Hormone and Cortisol are released in “pulses” or only when the body is under stress.

A random GH concentration is not a useful standalone diagnostic test. Similarly, a single morning cortisol can be informative but may not exclude central adrenal insufficiency. To get a definitive answer, doctors use stimulation tests (also called dynamic tests). During these tests, your child is given a medication that “challenges” the pituitary to produce a hormone. Doctors then take blood samples over time to see if the gland can rise to the occasion ^[7][8]. Test choice, timing, and safety are highly individualized; never withhold medications prior to a test unless instructed by your endocrinologist.

The Role of Genetic Testing

Your doctor may offer genetic testing to see if there is a specific mutation causing the PSIS. While this can provide answers for some families, it is important to manage expectations:

• Variable Yield: No single cause is identified in many patients. A negative panel does not exclude a genetic or multifactorial cause ^[9][4].
• Syndromic Clues: Genetic testing is most helpful if your child has other findings, such as vision problems or extra fingers/toes. Genes like HESX1, LHX4, and OTX2 are sometimes involved ^[10][4].
• Future Planning: Finding a genetic cause can help with genetic counseling, though most PSIS cases are “sporadic” ^[2][11].

A Checklist for Parents

Ensure your endocrinology team has evaluated these key areas, especially during the first year of diagnosis:

1. Morning Cortisol and ACTH (to screen the life-critical adrenal axis, though a single morning cortisol may not exclude central adrenal insufficiency) ^[12].
2. Free T4 and TSH (to check thyroid function) ^[12].
3. IGF-1 (a supportive marker for growth hormone levels) ^[12].
4. Electrolytes (to check sodium levels and water balance) ^[13].
5. Growth Velocity (tracking height on a specialized chart over several months) ^[14].

Because PSIS can be “evolving,” a “normal” test today does not mean the axis will always be normal. Regular monitoring is the most important part of the diagnostic process ^[15][16].