Week 26: HPO Axis

Question 1

At pt demonstrates elevated prolactin. What is the order of investigation?

Answer 1

1. Rule out pregnancy
2. Repeat prolactin level to rule out misreading due to stress/satiation/exercise
3. Obtain detailed drug history to rule out medication effect
4. Ensure no renal insufficiency
5. If prolactin is persistently elevated, image with MRI or CT if MRI is not available.

Question 2

Why isn’t GH a good test?

Answer 2

Released in a pulsatile fashion. Use IGF-1 instead.

Question 3

24 hour urine cortisol.

Answer 3

Screen for Cushing’s.

• integrated measurement of free cortisol
• The higher the value (2-3 times more than normal), the more certain of Cushing’s
• results can be normal in early/mild disease

Question 4

How should we screen for Cushings?

Answer 4

1. 24 hour urine cortisol.

2. Overnight dexamethasone suppression test

Question 5

Overnight dexamethasone test

Answer 5

Screen for Cushing’s.

• Bolus of dexamethasone at midnight and measure serum cortisol at 8am
• The lower the measured cortisol, the more certain we can be at ruling out cushing’s

Question 6

When might overnight dexamethasone test give you a false positive?

Answer 6

Pseudo-Cushings; 
Increased CBG (OCP, estrogen, pregnancy); 
Lower dexamethasone levels (poor absorption, drugs that induce metabolism);

Question 7

Pituitary incidentaloma

Answer 7

10% of normal people have hypointensities that are consistent with pituitary adenoma.

Question 8

What to do if you discover a pituitary incidentaloma

Answer 8

Scan to see if it is functioning.
If it is a macroadenoma, scan for hypopituitarism and check visual fields if it is close to the optic chiasm.
Follow up imaging in the next year.

Question 9

Why is it important to establish a biochemical and clinical diagnosis before imaging?

Answer 9

10% of the population has an incidentaloma. If you jump to imaging you might find one of those and be misled!

Question 10

First line therapy for prolactin-producing tumors?

Answer 10

dopamine agonists

Question 11

First line treatment for functioning pituitary tumours (other than a prolactin-secreting tumor) and symptomatic non-functioning tumor

Answer 11

pituitary surgery

Question 12

common causes of goitre in order

Answer 12

1. iodine deficiency
2. Hashimotos
3. Graves
4. multinodular disease

Question 13

Thyroid adenoma

Answer 13

a benign neoplasm derived from follicular cells. More common in women
older than 30.
Do not require further workup unless they are growing or compressive.

Question 14

How does prolactin cause testosterone insufficiency?

Answer 14

Prolactin inhibits GnRH, thus interfering with release of LH and FSH. LH is responsible for stimulating Leydig cells to synthesize testosterone from cholesterol (a series of steps)

Question 15

What are the risks of the oral glucose tolerance test? When is it contraindicated?

Answer 15

Risk of hypoglycemia. Absolutely contraindicated in patients with chronic heart disease, cerebrovascular disease, and epilepsy.