What is the diagnosis and management of radiation thyroiditis in patients with a history of head or neck cancer, including thyroid cancer, lymphoma, or other head and neck cancers, who have undergone radiation therapy?

Radiation Thyroiditis: Diagnosis and Management

Overview and Clinical Presentation

Radiation thyroiditis is a common acute inflammatory complication of head and neck radiotherapy that manifests during treatment, typically after cumulative doses of ≥12 Gy, and can progress to chronic hypothyroidism in 20-65% of patients depending on treatment modality. 1, 2

The condition presents in two distinct phases:

Acute Phase (During Radiotherapy)

• Transient hyperthyroid symptoms occur during active radiation treatment due to inflammatory destruction of thyroid follicles and release of stored thyroid hormone 1
• Biochemical changes include:
  • Decreased TSH levels (p < 0.0001) 1
  • Elevated FT4 and T4 levels (p < 0.0001 and p < 0.022 respectively) 1
  • Threshold dose for significant changes: 12 Gy (BED₂) 1
• Ultrasonographic findings demonstrate hypoechogenicity, irregular echo patterns, and increased vascularity with elevated pulsatility index, resistive index, and peak systolic velocity in the inferior thyroid artery 1

Chronic Phase (Post-Radiotherapy Hypothyroidism)

• Onset ranges from 4 weeks to 5-10 years post-treatment, with incidence increasing over time 3, 4
• Overall incidence: 20-48% after radiation alone 3, 2
• Incidence increases dramatically to 65% when radiotherapy is combined with surgery involving partial thyroidectomy 2

Diagnosis

Baseline Assessment

Obtain thyroid function studies (TSH, FT4, T4, FT3, T3) prior to initiating radiation therapy to establish baseline values 5, 3

Surveillance Schedule

• TSH monitoring every 6-12 months during the first 2 years post-radiation 5
• Extended monitoring at 1,2, and 5 years post-treatment for patients with thyroid exposure in the radiation field 6
• Continue annual monitoring beyond 5 years, as late-onset hypothyroidism can occur up to 10 years post-treatment 3

Clinical Manifestations Requiring Immediate Evaluation

• Severe hypothyroidism can cause massive head, neck, and hypopharyngeal edema 2
• Poor wound healing and bleeding disorders 3
• Resistant pharyngeal fistulas that fail standard management 7
• Skin flap edema and necrosis 7

Risk Factors

Higher-risk patients requiring more intensive monitoring include: 4, 2

• Female sex
• Small thyroid volume
• Previous neck surgery (especially partial thyroidectomy)
• Higher radiation doses to thyroid gland
• Combined modality treatment (surgery + radiation)

Management

Acute Radiation Thyroiditis (During Treatment)

• Supportive care is the primary approach, as acute thyroiditis is self-limited 1
• Monitor for symptomatic hyperthyroidism from hormone release
• Continue radiation therapy as planned unless severe symptoms develop

Subclinical Hypothyroidism (Elevated TSH, Normal FT4)

Initiate levothyroxine replacement therapy for subclinical hypothyroidism in head and neck cancer patients, as this prevents progression to overt hypothyroidism and associated complications including poor wound healing and resistant fistulas 2, 7

Overt Hypothyroidism

• Prompt hormonal replacement therapy is essential and may be the keystone in resolving treatment-resistant complications such as pharyngeal fistulas 7
• Standard levothyroxine dosing with titration based on TSH levels
• Hormonal replacement can reverse adverse physiological outcomes when initiated promptly 3

Prevention Strategies

Radiation Planning

• Utilize intensity-modulated radiotherapy (IMRT) to limit thyroid dose and reduce hypothyroidism incidence 4
• Document planned radiation dose to thyroid gland in treatment planning 4
• Consider thyroid-sparing techniques when oncologically safe

Patient Education

• Inform patients about 20-65% risk of hypothyroidism depending on treatment modality 3, 2
• Emphasize importance of long-term thyroid function monitoring
• Educate about symptoms requiring immediate evaluation

Critical Clinical Pitfalls

Do not dismiss subclinical hypothyroidism as clinically insignificant – the majority of post-radiation hypothyroidism presents as elevated TSH with normal FT4, and untreated subclinical disease can lead to serious complications including resistant fistulas and wound healing problems 2, 7

Do not assume euthyroid status based on clinical examination alone – patients may be clinically euthyroid despite biochemical hypothyroidism, and complications can develop before overt clinical symptoms appear 7

Do not discontinue monitoring after 2 years – hypothyroidism can develop 5-10 years post-treatment, requiring lifelong surveillance 3

Special Considerations

There is emerging evidence suggesting a potential protective role of hypothyroidism with respect to disease control and survival in head and neck cancers, though this requires further investigation before influencing clinical practice 3