Testing and measuring disease endocrine functions

TESTING ENDOCRINE FUNCTION

Endocrine function is assessed by measurement of hormone levels in blood (or more precisely in plasma or serum) and sometimes in other body fluids on samples obtained basally and in response to stimulation and suppression tests.

Basal blood levels

Assays for all clinically relevant pituitary and end-organ hormones are available.

The time, day and condition of measurement make great differences to hormone levels, and the method and timing of samples therefore depends upon the characteristics of the endocrine system involved. There are also sex, developmental and age differences.

Basal levels are especially useful for systems with long half-lives (e.g. T₄ and T₃, IGF-1, androstenedione, SHBG). These vary little over the short term and random samples are therefore satisfactory.

Basal samples for many hormones need to be interpreted with respect to normal ranges for the time of day/month, diet or posture concerned. Hormones with a marked circadian rhythm (testosterone in men, cortisol, ACTH, 17αOH-progesterone) must be measured at appropriate time of day (typically at 08:00 to 10:00 but, e.g. at 24:00 to demonstrate normal low levels of cortisol at this time). LH/FSH, oestrogen and progesterone vary with time of menstrual cycle and renin/aldosterone vary with sodium intake, posture and age. For these hormones, all relevant details must be recorded or the results may prove uninterpretable.

Stress-related hormones

Measurement of stress-related hormones may be problematic either because the patient is stressed by hospital attendance or venepuncture, leading to falsely high levels (e.g. catecholamines, prolactin where sampling via an indwelling needle some time after initial venepuncture may be required) or because low levels in a non-stressed individual are unable to confirm an adequate reserve required for normal physiological stress (cortisol and GH).

Urine collections

Collections over 24 hours have the advantage of providing an ‘integrated mean’ of a day’s secretion but in practice are often incomplete or wrongly timed. They also vary with sex and body size or age. Written instructions should be provided for the patient to ensure accurate collection. Examples of hormones measured in this way are catecholamines and urinary free cortisol levels.

Saliva

Saliva is sometimes used for steroid estimations, especially in children or for samples taken at home. Midnight salivary cortisol levels are increasingly used for the diagnosis of Cushing’s syndrome due to the practical difficulties in obtaining a midnight blood sample.

Stimulation and suppression tests

These tests are used when basal levels give equivocal information. In general, stimulation tests are used to confirm suspected deficiency, and suppression tests to confirm suspected excess of hormone secretion. These tests are valuable in many instances.