Interpretation of biochemical results may be heavily influenced by fasting status (e.g. triglycerides), sample obtained under basal conditions or pharmacologically stimulated (e.g. growth hormone), time of day (e.g. testosterone level in pubertal boy), age and pubertal status (e.g. IGF-1 level), stage of menstrual cycle (e.g. progesterone level), menopausal status (e.g. gonadotrophin levels), pregnancy (e.g. thyroid function tests), stress associated with sample collection (e.g. prolactin), and medication (e.g. exogenous oestrogen on cortisol levels).
Numerous endocrine tests have efficacy (combination of specificity and sensitivity) calculated for the investigation of a specific endocrine disorder, deficiency, or aetiology. The helpfulness of such endocrine tests in delineating the cause of a disorder is influenced by the underlying epidemiology of distribution of causes; for example ACTH-secreting pituitary adenomas are responsible for approximately 80% of adult cases of ACTH-dependent Cushings syndrome. Thus a test aimed at differentiating between the various causes of ACTH-dependent CS requires an efficacy of greater than 80% to make a significant contribution. Furthermore when investigating the aetiology of ACTH-dependent CS, inferior petrosal sinus sampling (IPSS) is often required. If the CS is cyclical, it is crucial that IPSS is not carried out when the disease is inactive, otherwise central: peripheral ACTH ratios are obtained which may be mis-interpreted as pituitary disease but in reality reflect normality.
The degree of elevation of the prolactin level in a patient with a pituitary macroadenoma, if interpreted in the context of the size of the macroadenoma provides important information as to whether the adenoma is a prolactinoma or non-functioning associated with disconnection hyperprolactinaemia.
In patients suspected of hypopituitarism, a knowledge of the natural evolution of pituitary hormone deficiencies will often allow one to predict the extent of the hypopituitarism. If TSH deficient suspect ACTH deficiency; if normal growth hormone status then high chance remainder of pituitary function normal.