Endocrinology

--

Hormone Classification and Signaling

Types of Hormones

Hormones can be classified by their chemical structure and mechanism of action:

Hormone Signaling Pathways

# Second messenger systems
second_messengers = {
    'cAMP': {
        'activator': 'Gs protein-coupled receptors',
        'effectors': ['PKA', 'CREB'],
        'cascade': 'Adenylyl cyclase → cAMP → PKA → Phosphorylation'
    },
    'IP3/DAG': {
        'activator': 'Gq protein-coupled receptors',
        'effectors': ['PKC', 'Ca²⁺ release'],
        'cascade': 'PLC → IP3 + DAG → Ca²⁺/PKC activation'
    },
    'cGMP': {
        'activator': 'Natriuretic peptides, NO',
        'effectors': ['PKG'],
        'cascade': 'Guanylyl cyclase → cGMP → PKG'
    }
}

# Nuclear receptor signaling
nuclear_receptors = {
    'Type I': {
        'location': 'Cytoplasm',
        'examples': ['GR', 'MR', 'AR', 'ER', 'PR'],
        'mechanism': 'Bind hormone in cytoplasm, translocate to nucleus'
    },
    'Type II': {
        'location': 'Nucleus',
        'examples': ['TR', 'RAR', 'VDR', 'PPAR'],
        'mechanism': 'Pre-bound to DNA, recruit co-activators on hormone binding'
    }
}

Feedback Loops

Endocrine systems use sophisticated feedback mechanisms:

Negative Feedback (most common):

• HPA axis: Cortisol suppresses CRH and ACTH
• HPT axis: Thyroid hormone suppresses TSH
• Glucose-insulin: High glucose stimulates insulin, which lowers glucose

Positive Feedback (rare):

• Oxytocin during labor: Contractions stimulate more oxytocin
• LH surge: Estrogen stimulates LH release

Major Endocrine Glands

Pituitary Gland

The pituitary has anterior (adenohypophysis) and posterior (neurohypophysis) portions:

# Pituitary hormones
anterior_pituitary = {
    'GH': {
        'name': 'Growth Hormone',
        'target': 'Liver, bone, muscle',
        'action': 'Growth, metabolism, IGF-1 production',
        'dysfunction': 'Acromegaly (excess), dwarfism (deficiency)'
    },
    'PRL': {
        'name': 'Prolactin',
        'target': 'Mammary gland',
        'action': 'Milk production',
        'dysfunction': 'Galactorrhea, infertility'
    },
    'ACTH': {
        'name': 'Adrenocorticotropic Hormone',
        'target': 'Adrenal cortex',
        'action': 'Cortisol synthesis',
        'dysfunction': 'Cushing disease (excess), Addison (deficiency)'
    },
    'TSH': {
        'name': 'Thyroid Stimulating Hormone',
        'target': 'Thyroid',
        'action': 'Thyroid hormone synthesis',
        'dysfunction': 'Hyper/hypothyroidism'
    },
    'LH/FSH': {
        'name': 'Gonadotropins',
        'target': 'Gonads',
        'action': 'Sex steroid production, gametogenesis',
        'dysfunction': 'Infertility'
    }
}

posterior_pituitary = {
    'ADH/vasopressin': {
        'action': 'Water retention, vasoconstriction',
        'dysfunction': 'DI (deficiency), SIADH (excess)'
    },
    'Oxytocin': {
        'action': 'Uterine contraction, milk ejection',
        'dysfunction': 'Impaired labor, bonding issues'
    }
}

Thyroid Gland

# Thyroid hormone synthesis
thyroid_hormones = {
    'T4 (thyroxine)': {
        'full_name': '3,5,3',5'-tetraiodothyronine',
        'binding': '99.97% protein-bound (TBG)',
        'half_life': '7 days',
        'activity': 'Prohormone, converted to T3'
    },
    'T3 (triiodothyronine)': {
        'full_name': '3,5,3'-triiodothyronine',
        'binding': '99.5% protein-bound (TBG)',
        'half_life': '1 day',
        'activity': 'Active form'
    },
    'rT3 (reverse T3)': {
        'binding': 'Low',
        'activity': 'Inactive, increased in illness'
    }
}

# Synthesis pathway
synthesis_steps = [
    '1. Iodide uptake via NIS (sodium-iodide symporter)',
    '2. Oxidation of iodide by thyroid peroxidase (TPO)',
    '3. Organification:iodination of tyrosine residues on thyroglobulin',
    '4. Coupling of MIT and DIT to form T3 and T4',
    '5. Storage in colloid',
    '6. Proteolysis and release'
]

Adrenal Gland

# Adrenal cortex zones and hormones
adrenal_zones = {
    'Zona glomerulosa': {
        'hormones': ['Aldosterone'],
        'regulator': 'Renin-angiotensin system, K⁺',
        'function': 'Mineralocorticoid: Na⁺ retention, K⁺ excretion'
    },
    'Zona fasciculata': {
        'hormones': ['Cortisol'],
        'regulator': 'ACTH',
        'function': 'Glucocorticoid: metabolism, stress response'
    },
    'Zona reticularis': {
        'hormones': ['Androgens (DHEA, androstenedione)'],
        'regulator': 'ACTH',
        'function': 'Precursors to sex steroids'
    }
}

adrenal_medulla = {
    'hormones': ['Epinephrine', 'Norepinephrine'],
    'regulator': 'Sympathetic preganglionic neurons',
    'function': 'Fight-or-flight response'
}

Metabolic Disorders

Diabetes Mellitus

# Diabetes classification and diagnosis
diabetes_diagnosis = {
    'Type 1 Diabetes': {
        'cause': 'Autoimmune destruction of β-cells',
        'insulin': 'Absolute deficiency',
        'age_onset': 'Usually < 30 years',
        'ketosis': 'Prone to DKA',
        'c_peptide': 'Low/undetectable',
        'treatment': 'Insulin therapy'
    },
    'Type 2 Diabetes': {
        'cause': 'Insulin resistance + relative insulin deficiency',
        'insulin': 'Relative deficiency',
        'age_onset': 'Usually > 40 years',
        'ketosis': 'Usually not',
        'c_peptide': 'Normal or high',
        'treatment': 'Lifestyle, OHA, insulin'
    },
    'Gestational Diabetes': {
        'cause': 'Pregnancy-induced insulin resistance',
        'timing': '24-28 weeks gestation',
        'risk_factors': ['Obesity', 'PCOS', 'Family history'],
        'treatment': 'Diet, insulin if needed'
    },
    'MODY': {
        'cause': 'Genetic defects in β-cell function',
        'inheritance': 'Autosomal dominant',
        'subtypes': ['HNF1α', 'GCK', 'HNF4α', etc.]
    }
}

# Diagnostic criteria
diagnostic_criteria = {
    'FPG': '≥126 mg/dL (7.0 mmol/L)',
    '2h OGTT': '≥200 mg/dL (11.1 mmol/L)',
    'HbA1c': '≥6.5% (48 mmol/mol)',
    'RPG': '≥200 mg/dL with classic symptoms',
    'Prediabetes FPG': '100-125 mg/dL',
    'Prediabetes 2h': '140-199 mg/dL',
    'Prediabetes HbA1c': '5.7-6.4%'
}

Thyroid Disorders

# Hypothyroidism
hypothyroidism = {
    'Primary': {
        'cause': 'Thyroid gland failure',
        'TSH': 'High',
        'FT4': 'Low',
        'causes': ['Hashimoto's', 'post-thyroidectomy', 'radiation']
    },
    'Secondary': {
        'cause': 'Pituitary failure',
        'TSH': 'Low/normal',
        'FT4': 'Low',
        'causes': ['Pituitary tumor', 'Sheehan syndrome']
    },
    'Tertiary': {
        'cause': 'Hypothalamic failure',
        'TSH': 'Low',
        'FT4': 'Low',
        'causes': ['Hypothalamic disease']
    }
}

# Hyperthyroidism
hyperthyroidism = {
    'Graves'': {
        'cause': 'TSH receptor antibodies (TRAb)',
        'TSH': 'Suppressed',
        'FT4/FT3': 'Elevated',
        'features': ['Graves ophthalmopathy', 'pretibial myxedema']
    },
    'Toxic multinodular': {
        'cause': 'Autonomous thyroid nodules',
        'TSH': 'Suppressed',
        'FT4/FT3': 'Elevated',
        'features': ['Multiple hot nodules on scan']
    },
    'Thyroiditis': {
        'cause': 'Inflammation releasing stored hormone',
        'TSH': 'Suppressed initially',
        'FT4/FT3': 'Elevated initially',
        'types': ['Subacute', 'Hashimoto's', 'Postpartum']
    }
}

Adrenal Disorders

# Cushing syndrome
cushing = {
    'Etiology': {
        'ACTH-producing pituitary': '70% (Cushing disease)',
        'Ectopic ACTH': '15%',
        'Adrenal adenoma/carcinoma': '15%'
    },
    'Diagnosis': {
        'screening_tests': ['24h UFC', 'Late-night saliva cortisol', 'DST'],
        'localization': ['ACTH level', 'High-dose DST', 'Imaging']
    },
    'clinical_features': [
        'Central obesity', 'Moon face', 'Buffalo hump',
        'Purple striae', 'Hypertension', 'Hyperglycemia',
        'Osteoporosis', 'Psychiatric changes'
    ]
}

# Addison disease
addison = {
    'cause': 'Primary adrenal insufficiency',
    'autoimmune': '70-90% in developed countries',
    'diagnostic_tests': {
        'baseline_cortisol': 'Low',
        'ACTH': 'High',
        'aldosterone': 'Low',
        'adrenal_autoantibodies': 'Often positive'
    },
    'clinical_features': [
        'Fatigue', 'Hyperpigmentation', 'Hypotension',
        'Salt craving', 'Weight loss', 'Nausea'
    ],
    'adrenal_crisis': 'Life-threatening, need stress steroids'
}

Metabolic Syndrome

# Metabolic syndrome criteria (NCEP ATP III)
metabolic_syndrome = {
    'criteria': {
        'waist_circumference': {
            'male': '>102 cm (>40 in)',
            'female': '>88 cm (>35 in)'
        },
        'triglycerides': '≥150 mg/dL',
        'HDL cholesterol': {
            'male': '<40 mg/dL',
            'female': '<50 mg/dL'
        },
        'blood_pressure': '≥130/85 mmHg',
        'fasting_glucose': '≥100 mg/dL'
    },
    'diagnosis': '3 or more criteria',
    'components': [
        'Central obesity',
        'Dyslipidemia',
        'Hypertension',
        'Insulin resistance'
    ],
    'treatment': {
        'lifestyle': 'Weight loss, exercise, diet',
        'pharmacologic': 'Metformin, statins, antihypertensives'
    }
}

Calcium Metabolism

# Calcium regulation
calcium_regulation = {
    'normal_range': '8.5-10.5 mg/dL (2.12-2.62 mmol/L)',
    'ionized_ca': '4.5-5.3 mg/dL (1.12-1.32 mmol/L)',
    
    'parathyroid_hormone': {
        'secretion_trigger': 'Low ionized Ca²⁺',
        'actions': [
            'Increase bone resorption',
            'Increase kidney Ca²⁺ reabsorption',
            'Increase vitamin D activation',
            'Decrease phosphate reabsorption'
        ]
    },
    'calcitonin': {
        'secretion_trigger': 'High Ca²⁺',
        'actions': ['Inhibit bone resorption', 'Increase Ca²⁺ excretion']
    },
    'vitamin_D': {
        'forms': ['D2 (ergocalciferol)', 'D3 (cholecalciferol)'],
        'activation': 'Liver → Kidney (1,25(OH)₂D)',
        'actions': [
            'Increase intestinal Ca²⁺ absorption',
            'Increase bone resorption',
            'Increase kidney Ca²⁺ reabsorption'
        ]
    }
}

# Disorders
hypercalcemia = {
    'causes': ['Primary hyperparathyroidism', 'Malignancy', 'Vitamin D excess'],
    'symptoms': ['Stones', 'Bones', 'Moans', 'Psychiatric'],
    'treatment': ['IV fluids', 'Bisphosphonates', 'Calcitonin', 'Surgery']
}

hypocalcemia = {
    'causes': ['Hypoparathyroidism', 'Vitamin D deficiency', 'Chronic kidney disease'],
    'symptoms': ['Tetany', 'Seizures', 'Arrhythmias', 'Paresthesias'],
    'treatment': ['IV calcium gluconate', 'Vitamin D', 'Calcium supplements']
}

Hormone Replacement Therapy

Types of HRT

Glucocorticoid Replacement

# Glucocorticoid dosing
glucocorticoid_equivalencies = {
    'hydrocortisone': '20 mg',
    'prednisone': '5 mg',
    'prednisolone': '5 mg',
    'dexamethasone': '0.5 mg',
    'methylprednisolone': '4 mg'
}

# Stress dosing protocol
stress_dosing = {
    'minor_illness': 'Double usual dose for 2-3 days',
    'major_stress': '50-100 mg hydrocortisone IV q6h',
    'surgery': '100 mg hydrocortisone IV pre-op, then 50 mg q8h',
    'diarrhea/vomiting': 'IV hydrocortisone, not oral'
}

Laboratory Evaluation

Endocrine Testing

# Dynamic function tests
dynamic_tests = {
    'ACTH_stimulation_test': {
        'purpose': 'Assess adrenal reserve',
        'method': '250 μg cosyntropin IV',
        'normal': 'Cortisol >18 μg/dL at 30-60 min',
        'interpretation': 'Peak <18 = adrenal insufficiency'
    },
    'CRH_stimulation_test': {
        'purpose': 'Differentiate Cushing causes',
        'method': '1 μg/kg CRH IV',
        'pituitary': 'ACTH and cortisol rise',
        'ectopic': 'No response',
        'adrenal': 'Cortisol rise, no ACTH'
    },
    'Insulin_tolerance_test': {
        'purpose': 'Assess GH and cortisol axis',
        'method': '0.1-0.15 U/kg insulin IV',
        'normal': 'GH >5 μg/L, cortisol >18 μg/dL',
        'contraindications': 'Seizures, heart disease'
    },
    'Oral_glucose_tolerance_test': {
        'purpose': 'Diagnose diabetes, assess insulin resistance',
        'method': '75g glucose PO',
        'diabetes': '2h glucose >200 mg/dL',
        'prediabetes': '140-199 mg/dL'
    }
}

# Hormone reference ranges
reference_ranges = {
    'cortisol': {
        'morning': '5-25 μg/dL',
        'evening': '<10 μg/dL'
    },
    'TSH': '0.4-4.0 mIU/L',
    'free_T4': '0.8-1.8 ng/dL',
    'free_T3': '2.3-4.2 pg/mL',
    'insulin': '2-25 μIU/mL',
    'HbA1c': '<5.6% (normal), 5.7-6.4% (prediabetes), ≥6.5% (diabetes)'
}

Common Errors to Avoid

• Ignoring feedback loops: Endocrine systems are tightly regulated; understand the axis
• Treating lab values in isolation: Always interpret in clinical context
• Missing secondary/tertiary causes: Don't assume primary pathology without testing
• Overtreating subclinical disease: Not all abnormal values require intervention
• Ignoring diurnal variation: Cortisol, ACTH have circadian rhythms
• Forgetting drug interactions: Many medications affect hormone levels
• Neglecting stress doses: Patients on chronic steroids need stress coverage
• Using total instead of free hormones: Protein binding affects interpretation
• Assuming thyroid antibodies are always significant: Context matters
• Not considering assay limitations: Different assays have different reference ranges