UNDERSTANDING LABORATORY TESTS USED IN ENDOCRINOLOGY

Understanding How Hormonal Lab Tests Are Used to Put a Clinical Diagnosis

Hormonal lab tests are critical tools in diagnosing endocrine disorders because they allow clinicians to measure hormone levels and assess the functionality of endocrine glands. These tests provide quantitative data on hormone concentrations, which can indicate whether a gland is overproducing (hyperfunction) or underproducing (hypofunction) hormones. Additionally, hormonal lab tests help identify underlying pathophysiological mechanisms, such as primary gland dysfunction or secondary causes due to upstream regulatory issues (e.g., pituitary or hypothalamic dysfunction).

For example:

1. Thyroid Function Testing: Measurement of thyroid-stimulating hormone (TSH), free T4, and free T3 levels helps diagnose conditions like hypothyroidism or hyperthyroidism. Elevated TSH with low free T4 suggests primary hypothyroidism, while suppressed TSH with high free T3 and/or free T4 indicates hyperthyroidism.
2. Adrenal Function Testing: Basal cortisol levels combined with adrenocorticotropic hormone (ACTH) measurements can differentiate between primary adrenal insufficiency (Addison’s disease) and secondary adrenal insufficiency caused by pituitary dysfunction.
3. Reproductive Hormone Testing: Measuring luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone, estrogen, and progesterone aids in diagnosing conditions such as polycystic ovary syndrome (PCOS), hypogonadism, or premature ovarian insufficiency.

These static tests often serve as the first step in identifying abnormalities but may not always provide definitive answers due to overlapping normal ranges or compensatory mechanisms.

Examples of the Role of Hormonal Lab Tests in Formulating Endocrine Diagnoses

1. Cushing’s Syndrome Diagnosis:
   • Cushing’s syndrome involves excess cortisol production. Initial screening includes measuring 24-hour urinary free cortisol or late-night salivary cortisol levels. Elevated results suggest hypercortisolism.
   • To confirm the diagnosis and identify its cause, dynamic suppression testing using dexamethasone is employed (see below for details).

2. Diabetes Mellitus Diagnosis:
   • Blood glucose levels and glycated hemoglobin (HbA1c) are used to diagnose diabetes mellitus. A fasting plasma glucose ≥126 mg/dL or HbA1c ≥6.5% confirms diabetes.
   • In cases where type 1 diabetes is suspected, additional testing for autoantibodies against pancreatic beta cells may be performed.

3. Growth Disorders:
   • For suspected growth hormone deficiency in children or adults, basal growth hormone levels are measured alongside insulin-like growth factor-1 (IGF-1). However, since growth hormone secretion is pulsatile, dynamic stimulation testing using agents like insulin or arginine is often required.

Importance of Dynamic Hormonal Testing in the Evaluation of Hormone Functions

Dynamic hormonal testing plays a pivotal role when static measurements fail to provide clear diagnostic information due to factors like diurnal variation in hormone secretion or compensatory feedback loops within the endocrine system.

Key Features of Dynamic Testing

Dynamic tests involve either stimulating an endocrine gland to assess its reserve capacity or suppressing it to evaluate autonomous activity:

• Stimulation Tests: These are used when hypofunction is suspected.
  • Example: The ACTH stimulation test evaluates adrenal function by administering synthetic ACTH and measuring subsequent cortisol production.
  • Another example is the GnRH stimulation test for assessing delayed puberty; it measures LH/FSH response after administration of gonadotropin-releasing hormone agonists.

• Suppression Tests: These are used when hyperfunction is suspected.
  • Example: The low-dose dexamethasone suppression test assesses whether cortisol secretion can be suppressed appropriately in cases of suspected Cushing’s syndrome.

Advantages of Dynamic Testing

1. Improved Diagnostic Accuracy: By evaluating how glands respond under controlled conditions, dynamic tests reduce false positives/negatives that might arise from single-point measurements.
2. Differentiating Primary vs Secondary Causes: For instance, ACTH stimulation can distinguish between primary adrenal insufficiency and secondary causes related to pituitary dysfunction.
3. Predicting Disease Progression/Treatment Needs: In children with delayed puberty, GnRH stimulation results can predict whether spontaneous pubertal development will occur without intervention.

Examples Highlighting Importance

1. ACTH Stimulation Test for Adrenal Insufficiency
   • This test involves administering synthetic ACTH intramuscularly or intravenously and measuring serum cortisol at baseline, 30 minutes, and 60 minutes post-injection.
   • A peak cortisol level <18 µg/dL suggests adrenal insufficiency.

2. GnRH Agonist Stimulation Test for Precocious Puberty
   • Subcutaneous injection of triptorelin stimulates gonadotrophs in the anterior pituitary to release LH/FSH.
   • A peak LH ≥5–8 IU/L confirms central precocious puberty.

3. Dexamethasone Suppression Test for Cushing’s Syndrome
   • Patients take low-dose dexamethasone overnight; morning serum cortisol levels are then measured.
   • Failure to suppress cortisol below a specific threshold indicates autonomous cortisol production.

In summary, dynamic hormonal testing provides critical insights into endocrine gland functionality that cannot be obtained through basal measurements alone.