Can a single thyroid lobe become enlarged?

Yes, a single thyroid lobe can absolutely become enlarged.

Unilateral thyroid lobe enlargement is a well-recognized clinical entity that occurs through several mechanisms: compensatory hypertrophy when the contralateral lobe is absent or non-functional, isolated nodular disease affecting one lobe, or asymmetric pathology in multinodular goiter.

Mechanisms of Single Lobe Enlargement

Compensatory Hypertrophy

When one thyroid lobe is congenitally absent (thyroid hemiagenesis) or surgically removed, the remaining lobe undergoes compensatory hypertrophy to maintain euthyroid status. Recent case reports demonstrate that a single functioning lobe can enlarge significantly—volumes of 9.5 grams have been documented in compensatory hypertrophy 1. This compensatory mechanism typically maintains normal thyroid function, though the enlarged lobe remains susceptible to nodular changes and potential functional exhaustion from chronic hyperstimulation 1, 2.

Unilateral Nodular Disease

A single lobe can develop:

• Solitary nodules requiring evaluation per standard protocols 3
• Multinodular changes confined to one lobe 4, 5
• Diffuse goiter affecting only one side 4
• Isolated hyperplasia that can present as a mass lesion 6

Clinical Evaluation Approach

When encountering unilateral thyroid enlargement, follow this systematic evaluation:

1. Initial Assessment

• Measure TSH as the first-line test 3
• If TSH is low: measure FT4 and FT3 to assess for autonomous function
• If TSH is elevated: measure TPOAb and FT4 to evaluate for autoimmune disease
• Perform high-resolution ultrasonography on all patients with palpable thyroid abnormalities 3

2. Ultrasound Characterization

The ultrasound must determine:

• Whether the contralateral lobe is present or absent (to identify hemiagenesis)
• Size, echogenicity, margins, and vascularity of the enlarged lobe
• Presence of discrete nodules versus diffuse enlargement
• Risk stratification using standardized classification 3:
  • High-risk features: marked hypoechogenicity, microcalcifications, irregular margins, taller-than-wide shape
  • Intermediate-risk features: slight hypoechogenicity, smooth margins
  • Low-risk features: spongiform appearance, predominantly cystic (>50%)

3. FNA Decision Algorithm

Based on the 2016 AACE/ACE guidelines 3:

Do NOT biopsy:

• Nodules <5 mm regardless of appearance
• Functioning nodules on scintigraphy (if performed)

Consider FNA for:

• High-risk nodules ≥10 mm
• Intermediate-risk nodules >20 mm
• Low-risk nodules only if >20 mm AND increasing in size
• Any nodule 5-10 mm with high-risk features PLUS suspicious lymph nodes, extrathyroid extension, or positive family history

Limit to 2 nodules maximum per biopsy session when multiple nodules are present 3.

Critical Pitfalls to Avoid

1. Mistaking Hemiagenesis for Pathology

The most important pitfall is misinterpreting congenital absence of one lobe as a surgical defect or assuming the enlarged contralateral lobe represents neoplasia 1, 2. Always verify whether both lobes are present on imaging before attributing asymmetry to disease.

2. Unnecessary Surgery

Do not perform diagnostic hemithyroidectomy on a solitary functioning lobe without clear indication, as this will result in permanent hypothyroidism requiring lifelong replacement 1. Conservative management with surveillance is appropriate for asymptomatic patients with hemiagenesis and benign-appearing nodules.

3. Over-biopsying Benign-Appearing Nodules

The guidelines explicitly recommend against FNA of small (<5 mm) or clearly benign-appearing nodules, even when they cause visible asymmetry 3. This prevents the cascade of unnecessary interventions that contributes to thyroid cancer overdiagnosis 7.

Management of Unilateral Enlargement

For benign nodules causing symptoms (compressive symptoms, cosmetic concerns):

• Consider surgery when local pressure symptoms are clearly associated with the nodule 3
• Percutaneous ethanol injection is first-line for relapsing cystic lesions 3
• Thermal ablation (laser or radiofrequency) may be considered for solid symptomatic nodules 3

For asymptomatic benign nodules:

• Clinical and ultrasound follow-up at 12 months, then 24 months if stable 3
• Repeat FNA only if volume increases >50% or suspicious features develop 3
• LT4 suppressive therapy is NOT recommended in euthyroid patients 3

For hemiagenesis with compensatory hypertrophy:

• Long-term surveillance is essential as the single lobe may develop nodular disease or functional exhaustion 1, 2
• Maintain euthyroid status; treat hypothyroidism if it develops
• Avoid surgery unless clearly indicated to prevent iatrogenic permanent hypothyroidism

The key principle is that unilateral thyroid enlargement represents a spectrum from normal anatomic variation (hemiagenesis with compensation) to pathologic processes requiring intervention, and distinguishing between these requires systematic ultrasound evaluation and judicious use of FNA based on established risk criteria.