Is synacthen test reliable for pituitary gland function?

Question

Hi doctor,

I was diagnosed with Cushing's disease, which caused a high level of cortisol before two years. The tumor size was 2 mm. I had surgery and the pituitary gland tumor was removed. Then, I am having a low level of cortisol and T4. After the operation, I am taking Hydrocortisone (10 mg or 5 mg daily) and Levothyroxine. My latest blood test showed cortisol (AM ) 141, cortisol after medicine 261 (reference range 171 nmol/L - 526 nmol/L) and 24 hours urine test showed 547 nmol (reference range 57.7 - 806.8). I had an appointment with my endocrinologist last month. I had a short synacthen test and the results as per attachments are here. May I know is this test accurate to check my pituitary gland function? If yes, how this test ensure my pituitary gland function well? If no, why this test is not suitable for me? What is the suitable alternative test?

Answer 1

Hi, Welcome to icliniq.com.

I understand your question. I have noted all your laboratory test results (attachment removed to protect patient identity). The only doubt I want to clear is about the medicine. Did you mention cortisol PM (Post meridiem) part as after medicine? Which was the medicine you took? Was it Hydrocortisone? If yes, then I will not rely on this test. The second discrepancy I see is that your PM cortisol comes high. In expected normalcy, it should have come low. Because physiologically, the cortisol reduces in the evening time. But here, (I assume) it would have been higher than morning because of medicine. The 24-hour urine cortisol can also be misleading if the Hydrocortisone tablet was taken. Regarding Synacthen test, the documented results are normal, but I will not rely on that test to stop the Hydrocortisone. Because it is not widely accepted and practiced in a neurosurgical fraternity. We do not do that on our patients. But, yes, endocrine specialists may be using it more frequently. Conclusively, I would request you to continue your Hydrocortisone in the same dose, which you were taking as of now. Reason being, your AM cortisol is on the lower side and your PM cortisol is normal. Your 24 hours urine cortisol is not reliable in this case scenario again, because of medicine. You do not need to undergo any newer tests and the tests done now are sufficient.