Dr Trevor A Howlett
Consultant Endocrinologist, Leicester Royal Infirmary

One consistent - probably unwelcome - part of a visit to an endocrinology clinic is that we will usually want yet another blood test, and sometimes a whole series of blood samples one after another! What do we do with all that blood - and how does it help your treatment?

Why blood tests?

Endocrinologists deal with disorders of hormones, and classical hormones are defined as substances which travel in the blood from an endocrine gland to have effects on other parts of the body. Therefore, if we want to know whether your hormones are working normally, or whether you are making too much or too little hormone, the only way to be sure is to measure levels in the blood. The pituitary controls an awful lot of hormones - hence a lot of blood tests!

What can blood tests tell us?

Simple blood tests

We can learn quite a lot about the way your Pituitary gland is working from a single blood sample - especially if it is taken at the right time of day.

Thyroid hormone levels are very stable from day to day and a single blood test can usually show whether or not you have a thyroid deficiency (when Thyroxine levels are below normal due to lack of pituitary Thyroid stimulating hormone - TSH), and/or whether you are on the correct amount of thyroxine replacement.

A single blood test for Prolactin can also distinguish whether levels are very high (which can be all we need to establish that your pituitary problem is a Prolactinoma), or high enough to need treatment with a tablet like cabergoline or Bromocriptine (for example to help irregular periods) or simply normal.

Growth hormone (GH) and IGF-1 levels can confirm whether or not your Acromegaly is under control - and when very high even a single sample can confirm a suspected diagnosis. A single measurement of IGF-1 levels can also be used to test whether patients on GH replacement therapy are on the correct amount of growth hormone replacement.

Sex hormone levels may be high enough to exclude a deficiency of these hormones - but here there is a catch since levels of all the relevant hormones go up and down. In women this variation is due to the monthly menstrual cycle and in men the male hormone level, Testosterone, is much higher first thing in the morning than it is in late afternoon or evening. This is one of the reasons why your endocrinologist may ask you to attend for a blood test at a particular time of the day (typically 9am) or of the month.

More complex diagnostic tests

Many hormone levels go up and down at different times of the day or month, or with illness or stress. In these cases a single blood sample often isn't enough to prove whether levels are normal, too high or too low. We may need to measure hormones several times through the day (to see the average), or after we try to stimulate it (if we suspect deficiency) or suppress it (if we suspect overactivity). These more complex tests, often called dynamic function tests, will often involve a needle which is left in the vein for a few hours. This needle is used to take blood samples throughout the duration of the test and can also be used to give an injection of a substance which we know should make hormone levels change. There are many possibilities but a few examples may help to give you the idea...

Steroid hormone (Cortisol) levels vary a lot in normal people - they are very low when you are asleep at midnight, high first thing in the morning, and need to go higher still to allow the body to cope with illnesses and other stress. Often a simple clinic blood sample cannot distinguish normality from mild deficiency or excess. If we want to rule out deficiency - and be sure that you will have enough steroid hormone for any stressful illness - then we will often need a stimulation test. Common examples are the insulin tolerance test (ITT), synacthen and glucagon tests - all of which normally make cortisol levels rise (choosing between these tests can have endocrinologists arguing for hours and is too complicated to explain here!). If we think levels might be too high we will often perform a dexamethasone suppression test after which levels become very low in normal people.

Growth hormone levels are high all through the day and night in patients with acromegaly - but normal people may have levels in the same range for short bursts during the day. A glucose tolerance test is usually performed to sort this out, since in normal people growth hormone suppresses to very low levels after glucose. Growth hormone deficiency can also cause a lot of problems, and we can only diagnose this with certainty if we prove that levels do not rise sufficiently after a stimulation test - typically again an insulin tolerance test or glucagon test.

If we want to be sure that levels of hormones or hormone replacement are correct throughout the day, we will often perform a 'day curve' for example Hydrocortisone (cortisol), growth hormone or Cushing's day curves.

Repeated tests

The state of your hormones may change with time. For example: pituitary surgery may improve or damage pituitary function; after Radiotherapy, hormone overactivity may slowly improve, but with radiotherapy and some progressive pituitary problems, deficiencies may also develop slowly over the course of many years. Tests therefore frequently need to be repeated at regular intervals - often annually, but sometimes more or less often.

Finally, if you don't understand why a simple or complex blood test is being performed then why not ask? Your endocrinologist or specialist nurse will almost certainly be happy to explain.