What are the clinical signs and laboratory findings that differentiate hypothyroidism from hyperthyroidism?

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Differentiating Hypothyroidism from Hyperthyroidism

The key distinction lies in opposing TSH and free T4 patterns: hypothyroidism presents with elevated TSH (>4.5 mIU/L) and low free T4, while hyperthyroidism shows suppressed TSH (<0.4 mIU/L) and elevated free T4 or T3.

Laboratory Findings

Hypothyroidism 1, 2

  • TSH: Elevated above 4.5 mIU/L (often >10 mIU/L in overt disease)
  • Free T4: Low (in overt hypothyroidism) or normal (in subclinical hypothyroidism)
  • Additional findings:
    • Elevated total cholesterol, LDL-cholesterol, and triglycerides
    • Elevated creatine kinase (MM fraction)
    • Possible elevation in liver enzymes
    • Hyponatremia (in severe cases)

Hyperthyroidism 3, 2

  • TSH: Suppressed below 0.4 mIU/L (often <0.1 mIU/L in overt disease)
  • Free T4 and/or T3: Elevated (in overt hyperthyroidism) or normal (in subclinical hyperthyroidism)
  • Note: If TSH is low but free T4 is normal, measure free T3 to identify T3-predominant hyperthyroidism 3

Clinical Signs and Symptoms

Hypothyroidism 1

Common presentations:

  • Dry skin
  • Cold intolerance
  • Fatigue and decreased activity
  • Muscle cramps and weakness
  • Voice changes (hoarseness)
  • Constipation
  • Weight gain
  • Hair loss
  • Prolonged ankle reflex relaxation time

Less common but important signs:

  • Carpal tunnel syndrome
  • Sleep apnea
  • Pituitary hyperplasia with or without hyperprolactinemia and galactorrhea
  • Hyponatremia
  • Bradycardia (decreased resting heart rate)
  • Macroglossia and myxedematous facies (severe cases)

Hyperthyroidism 2

Common presentations:

  • Unintentional weight loss
  • Heat intolerance
  • Palpitations and tachycardia
  • Tremor
  • Anxiety and restlessness
  • Hyperactivity
  • Diarrhea
  • Sweating

Additional findings:

  • Arrhythmias (including atrial fibrillation)
  • Goiter (may be present)
  • Ophthalmopathy (in Graves' disease)

Diagnostic Algorithm

Step 1: Measure TSH 3

  • If TSH is normal (0.4-4.5 mIU/L): Thyroid dysfunction is unlikely; no further testing needed unless high clinical suspicion
  • If TSH is elevated: Proceed to Step 2A
  • If TSH is suppressed: Proceed to Step 2B

Step 2A: Elevated TSH - Evaluate for Hypothyroidism 1, 2

  • Measure free T4
    • Low free T4: Overt hypothyroidism confirmed
    • Normal free T4: Subclinical hypothyroidism
  • Consider TPO antibodies to identify autoimmune etiology

Step 2B: Suppressed TSH - Evaluate for Hyperthyroidism 3, 2

  • Measure free T4
    • Elevated free T4: Overt hyperthyroidism confirmed
    • Normal free T4: Measure free T3
      • Elevated T3: T3-predominant hyperthyroidism
      • Normal T3: Subclinical hyperthyroidism

Critical Pitfalls to Avoid

In hypothyroidism:

  • When both adrenal insufficiency and hypothyroidism coexist, always start glucocorticoids before thyroid hormone replacement to prevent adrenal crisis 2
  • TSH may be falsely elevated in untreated adrenal insufficiency and normalize with glucocorticoid therapy 1
  • Central hypothyroidism presents with low/normal TSH and low free T4—do not rely on TSH alone if pituitary disease is suspected 1

In hyperthyroidism:

  • Distinguish thyroiditis (self-limiting, leads to hypothyroidism) from Graves' disease using TSH receptor antibodies and radioiodine uptake scan 2
  • Beta-blockers may mask hyperthyroid symptoms 2
  • Pregnancy, nonthyroidal illness, and certain medications (dopamine, glucocorticoids) can suppress TSH without true hyperthyroidism 4

General considerations:

  • Avoid thyroid function testing in hospitalized patients with acute illness unless there is strong clinical suspicion, as nonthyroidal illness can cause misleading results 1
  • Heterophilic antibodies and assay interference can cause falsely abnormal TSH or free T4 values 1

References

1
Guideline

clinical practice guidelines for hypothyroidism in adults: cosponsored by the american association of clinical endocrinologists and the american thyroid association.

Endocrine practice : official journal of the American College of Endocrinology and the American Association of Clinical Endocrinologist, 2012

3
Guideline

diagnostic procedure in suspected graves' disease.

Annales d'endocrinologie, 2018

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