What is the difference between hyperthyroidism and hypothyroidism?

Hyperthyroidism vs. Hypothyroidism: Key Differences

Hyperthyroidism and hypothyroidism represent opposite states of thyroid hormone dysregulation, with hyperthyroidism characterized by excess thyroid hormone causing increased metabolism, tachycardia, and weight loss, while hypothyroidism reflects thyroid hormone deficiency resulting in decreased metabolism, bradycardia, and weight gain. 1, 2

Fundamental Pathophysiologic Differences

Hyperthyroidism: Excess Thyroid Hormone State

Cardiovascular Effects:

• Increased heart rate and cardiac output (up to 300% above baseline), with decreased systemic vascular resistance 1
• Enhanced left ventricular contractility and isovolumic relaxation 1
• Risk of tachycardia-induced cardiomyopathy with persistent rapid rates from atrial fibrillation or flutter 1
• Pulmonary artery hypertension may develop despite systemic hypotension 1
• Cardiovascular complications are the chief cause of death in patients >50 years with hyperthyroidism 1

Metabolic and Hemodynamic Changes:

• Decreased systemic vascular resistance stimulates renin-angiotensin-aldosterone activation 1
• Blood volume increases up to 25% from enhanced erythropoietin release 1
• Increased preload with decreased afterload 1

Hypothyroidism: Thyroid Hormone Deficiency State

Cardiovascular Effects:

• Bradycardia, mild hypertension (often diastolic), and narrowed pulse pressure 1
• Low cardiac output from bradycardia, decreased ventricular filling, and reduced contractility 1
• Systemic vascular resistance increases up to 50% 1
• Slowed diastolic relaxation and filling 1
• Clinical heart failure is rare because cardiac output typically meets the lowered systemic demands 1

Structural Changes:

• Pericardial effusions and nonpitting edema (myxedema) occur in severe, long-standing cases 1
• Myocardial fibrosis develops through fibroblast stimulation 1
• Dilated cardiomyopathy is not a characteristic feature, though case reports exist 1

Laboratory Diagnosis

Hyperthyroidism Pattern:

• Suppressed TSH (typically <0.4 mIU/L) 3, 2
• Elevated free T4 and/or T3 in overt hyperthyroidism 2
• Normal T4/T3 with low TSH defines subclinical hyperthyroidism 2

Hypothyroidism Pattern:

• Elevated TSH (>4.5 mIU/L) 4, 5
• Low free T4 in overt hypothyroidism 4
• Normal T4 with elevated TSH defines subclinical hypothyroidism 4

Clinical Presentation Differences

Hyperthyroidism Symptoms:

• Anxiety, insomnia, tremor, heat intolerance 2
• Palpitations and tachycardia 3, 2
• Unintentional weight loss despite increased appetite 2
• Diarrhea 2
• Exophthalmos and stare in Graves' disease 2

Hypothyroidism Symptoms:

• Fatigue, cold intolerance 4, 5
• Weight gain 4, 5
• Constipation 4
• Cognitive slowing 4

Common Etiologies

Hyperthyroidism:

• Graves' disease (most common in developed countries, affecting 2% of women and 0.5% of men) 2, 6
• Toxic multinodular goiter and toxic adenoma 7, 2
• Thyroiditis with hormone leakage (Hashimoto's can cause transient hyperthyroidism) 3, 7
• Medication effects (amiodarone, dopamine, glucocorticoids) 3

Hypothyroidism:

• Autoimmune thyroiditis (Hashimoto's) - most common cause 5
• Iodine deficiency 5
• Post-treatment for hyperthyroidism 3
• Medication effects 3

Critical Management Differences

Hyperthyroidism Treatment:

• β-blockers as initial therapy to lower heart rate and improve symptoms 1
• Definitive treatment options: antithyroid drugs, radioactive iodine ablation, or surgery 2
• Treatment is generally recommended for TSH <0.1 mIU/L, particularly with Graves' disease or nodular thyroid disease 3

Hypothyroidism Treatment:

• Levothyroxine is the treatment of choice 8, 5
• Treatment mandatory for TSH >10 mIU/L regardless of symptoms 4
• For elderly patients (>70 years) or those with cardiac disease, start with lower doses (25-50 mcg/day) 4

Important Clinical Pitfalls

For Both Conditions:

• Thyroid function tests should be repeated over 3-6 months to confirm abnormal findings, as single measurements are insufficient 3
• Both conditions are rarely the primary cause of dilated cardiomyopathy, but commonly coexist with heart failure from other causes 1
• In older patients with underlying cardiac disease, the altered workload from either condition can precipitate heart failure 1

Hyperthyroidism-Specific:

• Undetectable TSH (<0.01 mIU/L) is rare in non-thyroidal illness unless patients receive concomitant glucocorticoids or dopamine 3
• Failure to consider non-thyroidal causes of low TSH in hospitalized or acutely ill patients leads to misdiagnosis 3

Hypothyroidism-Specific:

• Never start thyroid hormone before ruling out adrenal insufficiency in suspected central hypothyroidism, as this can precipitate adrenal crisis 4
• Approximately 25% of patients on levothyroxine are unintentionally maintained on excessive doses that suppress TSH, increasing risks for atrial fibrillation and osteoporosis 4