Principles of Endocrine Disease and its Assessment

Question 1

What law are bound and free hormones levels in the blood governed by?

Answer 1

Bound and free hormones are in equilibrium.
This is governed by the Law of Mass Action, which states that the rate of a chemical reaction is directly proportional to the products of the activities or the concentration of the reactants.

Question 2

Describe total hormone assays.

Answer 2

In these, we measure all the hormones in the bloodstream.
It is dependant on binding protein levels. However, it is not always reflective of the free hormones levels, as it measures everything.
It is cheap.

Question 3

To overcome problems faced by total hormone assays, we use free hormone assays.
Describe free hormone assays.

Answer 3

In these, we measure the amount of free hormone in the bloodstream. It is representative of the active hormone fraction. It is not binding protein dependent.
However, it is complex and expensive, and it involves a separation step. This means that it can be unreliable, and it can have poor reproducibility.

Question 4

Describe the separation step involved in free hormone assays.

Answer 4

In the separation step, we would use a dialysis membrane to ‘sieve’ out the free hormones. This membrane only allows the free hormones to cross it, meaning that we can measure the active hormone fraction.

However, as we move the free hormones, we alter the equilibrium. This means that more free hormone may pop up to balance out the change. This shows how the test can be unreliable.

To overcome this, you could use an ultracentrifuge, where you allow it to centrifuge, then you remove it from the solution. You could also add various precipitating agents to bind the proteins, which are then precipitated out so that you can measure the amount of free hormones.

Question 5

What problems can arise with assays due to DPP4, and how do we overcome this problem?

Answer 5

When the enzymes are done working, enzymes (DPP4) will cut them at specific motifs, switching them off. The protein fragments stay in the circulation (biologically inactive) until they are filtered out by the kidney.
However, these are measured as units of biological activity in assays when they are not.

To overcome this, we raise antibodies for the ends of the full biologically active protein. We then put that on the side of the test tube, let it sit to bind, then wash out all the unbound protein fragments. This gives you a more accurate picture.

Question 6

Describe how you would investigate different conditions of the thyroid gland.

Answer 6

OVERPRODUCTION:

• thyroid function test
• autoantibodies
• technesium scanning (functional assessment)

UNDERPRODUCTION:

• thyroid function test
• autoantibodies (mainly TPO)

THYROIDITIS (inflammation of the gland):

• thyroid function test
• ESR (erythrocyte sedimentation rate)
• technesium scanning

THYROID NODULE (small swelling):

• ultrasound scan
• FNA (fine needle aspiration) biopsy

Question 7

Describe the synacthen test.

Answer 7

Synacthen is synthetic ACTH. It can be used to test adrenal function.

SHORT SYNACTHEN TEST (250 mcg):

• used to diagnose primary adrenal failure
• blood taken over an hour

LONG SYNACTHEN TEST (1 mg):

• used to diagnose secondary adrenal failure (suspected pituitary problem, while adrenal is normal)
• blood taken over 24 hours