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Chemistry

Endocrinology

This section covers the biochemical basis, testing methods, interpretation of results, and associations with different disease states associated with endocrinology

Biochemical Theory and Pathways

  • Hormones as Messengers
    • Hormones are chemical messengers that regulate various physiological processes, including metabolism, growth, reproduction, and stress response
  • Hormone Synthesis, Secretion, and Transport
    • Hormones are synthesized in endocrine glands and secreted into the bloodstream in response to specific stimuli
    • They are transported either freely or bound to carrier proteins
  • Mechanisms of Action
    • Hormones bind to specific receptors on target cells, initiating intracellular signaling cascades that alter gene expression and cellular function
    • Receptors can be located on the cell surface (peptide hormones and catecholamines) or within the cell (steroid hormones and thyroid hormones)
  • Key Hormones and Their Functions
    • Pituitary Hormones: Growth hormone (GH), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), prolactin, luteinizing hormone (LH), follicle-stimulating hormone (FSH)
    • Thyroid Hormones: Thyroxine (T4), triiodothyronine (T3)
    • Adrenal Hormones: Cortisol, aldosterone, epinephrine, norepinephrine
    • Pancreatic Hormones: Insulin, glucagon
    • Parathyroid Hormone (PTH)
    • Gonadal Hormones: Testosterone, estradiol, progesterone
  • Feedback Mechanisms
    • Hormone levels are tightly regulated by negative feedback loops to maintain homeostasis

Laboratory Test Procedures

  • Principles of Hormone Measurement
    • Immunoassays: Based on antibody-antigen interactions (e.g., ELISA, CLIA, RIA)
    • Fluorescence Assays: Measure the intensity of emitted light from fluorescently labeled hormones
    • Liquid Chromatography-Mass Spectrometry (LC-MS): Separates and quantifies hormones based on their mass-to-charge ratio
  • Specimen Collection and Handling
    • Follow specific guidelines for patient preparation, collection tubes, order of draw, and specimen processing to ensure accurate results
  • Stimulation/Suppression Tests
    • Used to assess the functional reserve of endocrine glands and diagnose hormone excess or deficiency
    • Involve administration of stimulatory or suppressive agents and serial hormone measurements
  • Common Tests
    • Pituitary: GH, ACTH, TSH, prolactin, LH, FSH
    • Thyroid: Free T4, Free T3, TSH, thyroid antibodies
    • Adrenal: Cortisol, aldosterone, catecholamines
    • Parathyroid: PTH, calcium, vitamin D
    • Gonadal: Testosterone, estradiol, progesterone
    • Other: Insulin, C-peptide, human chorionic gonadotropin (hCG)

Test Result Interpretation

  • Reference Intervals
    • Use appropriate reference intervals for the specific assay and patient population
  • Clinical Context
    • Interpret test results in the context of the patient’s clinical history, physical examination findings, and other laboratory data
  • Multiple Tests
    • Assess the pattern of hormone levels rather than relying on a single value; dynamic tests may be needed
  • Interfering Substances
    • Be aware of potential interfering substances that can affect hormone measurements
  • Assay Limitations
    • Understand the limitations of the specific assay used, including its sensitivity, specificity, and potential for cross-reactivity
  • Common Findings
    • Elevated or suppressed hormone levels
    • Abnormal responses to stimulation/suppression tests
    • Presence of hormone-specific antibodies

Disease State Correlation

  • Pituitary Disorders
    • Acromegaly/Gigantism: Elevated GH and IGF-1
    • Cushing’s Disease: Elevated cortisol and ACTH
    • Hypopituitarism: Deficiencies in one or more pituitary hormones
  • Thyroid Disorders
    • Hyperthyroidism: Suppressed TSH and elevated free T4 and/or free T3
    • Hypothyroidism: Elevated TSH and low free T4
  • Adrenal Disorders
    • Cushing’s Syndrome: Elevated cortisol levels
    • Addison’s Disease (Primary Adrenal Insufficiency): Low cortisol and elevated ACTH
    • Pheochromocytoma: Elevated catecholamines and metanephrines
  • Parathyroid Disorders
    • Hyperparathyroidism: Elevated calcium and PTH
    • Hypoparathyroidism: Low calcium and PTH
  • Gonadal Disorders
    • Hypogonadism in Males: Low testosterone and elevated LH/FSH (primary) or low LH/FSH (secondary)
    • Polycystic Ovary Syndrome (PCOS): Elevated testosterone, LH/FSH ratio, and insulin resistance
  • Other Hormones
    • Diabetes Mellitus: Elevated glucose and HbA1c, variable insulin and C-peptide
    • Pregnancy: Elevated human chorionic gonadotropin (hCG)