Thyroiditis

Quick Answer

Thyroiditis causes thyrotoxicosis by releasing preformed hormone from an inflamed, damaged gland — not by overproduction — so radioiodine uptake (RAIU) is low, the mirror image of Graves disease. Recognizing this pattern prevents inappropriate I-131: there is no trapping tissue to treat. Management is supportive (beta-blockade; NSAIDs/steroids for the painful subacute form) through an often triphasic course (thyrotoxic → hypothyroid → recovery). Always exclude a recent iodine load and exogenous thyroid hormone, which also produce low-uptake thyrotoxicosis.

Background & Pathophysiology

Thyroiditis is destructive/inflammatory thyroid injury that spills stored T4/T3 into the circulation. Because the follicular cells are damaged and TSH is suppressed, iodine trapping falls — hence low RAIU. The subtypes:

• Subacute (de Quervain / granulomatous): often painful, post-viral, with elevated ESR/CRP; typically triphasic and self-limited.
• Painless / silent (a variant of autoimmune thyroiditis) and postpartum thyroiditis: painless, antibody-associated, often triphasic.
• Amiodarone-induced thyrotoxicosis type 2 (destructive; low uptake, low vascularity on Doppler) — contrast with type 1 (iodine-induced overproduction in nodular glands).
• Hashimoto (chronic lymphocytic): usually hypothyroid, but can have an early hashitoxicosis phase.

Clinical & Laboratory

Thyrotoxic symptoms (palpitations, tremor, heat intolerance) identical to Graves, but without ophthalmopathy/pretibial myxedema. Labs: suppressed TSH, elevated free T4/T3; elevated ESR/CRP and tender gland in subacute; TPO antibodies in autoimmune forms; a low or normal thyroglobulin is expected in factitious (exogenous) thyrotoxicosis, distinguishing it from true thyroiditis.

Imaging Role

The RAIU / pertechnetate scan sorts the thyrotoxicosis differential:

Color-flow Doppler complements uptake: low vascularity in destructive thyroiditis (and AIT type 2) vs the "thyroid inferno" hypervascularity of Graves.

Interpretation & Decision

A low RAIU in a thyrotoxic patient means the gland is leaking or loaded, not making — do not give I-131. Confirm there is no recent iodinated contrast/amiodarone/kelp, and consider exogenous hormone (check thyroglobulin — low in factitious). Anticipate the hypothyroid phase; most patients recover, though postpartum/silent forms may leave permanent hypothyroidism.

Differential Diagnosis

Graves disease (high uptake, orbitopathy), toxic nodular disease (focal uptake), exogenous/factitious thyrotoxicosis (low thyroglobulin), iodine-induced (Jod-Basedow), struma ovarii (pelvic uptake, absent neck uptake), and amiodarone type 1 vs type 2.

Reporting Checklist

• State the RAIU value with timing/reference range and the scan pattern (low/absent).
• Note Doppler vascularity if performed (low in destructive forms).
• Explicitly flag that low uptake is not a radioiodine indication; recommend supportive management.
• Document iodine-load/medication history.

Common Pitfalls

• Treating thyroiditis as Graves — wasted, ineffective radioiodine.
• Missing exogenous levothyroxine or iodine load as the cause of low-uptake thyrotoxicosis.
• Overlooking the hypothyroid phase and stopping surveillance early.
• Conflating amiodarone type 1 (iodine-induced, treat the overproduction) with type 2 (destructive, steroids).

Board Pearls

Thyroiditis causes thyrotoxicosis by releasing preformed hormone from an inflamed gland, not by overproduction — so RAIU is low, the mirror image of Graves. That single finding prevents inappropriate I-131 (nothing is trapping iodine to treat). Management is supportive (beta-blockade; NSAIDs/steroids for pain) through an often triphasic self-limited course.

Always exclude a recent iodine load and exogenous hormone, which also suppress uptake — check thyroglobulin (low in factitious/exogenous thyrotoxicosis). Doppler shows low vascularity in destructive thyroiditis vs the "thyroid inferno" of Graves.

Distinguish amiodarone-induced thyrotoxicosis type 2 (destructive, low uptake/low vascularity, steroid-responsive) from type 1 (iodine-induced overproduction in nodular glands). Consider struma ovarii when the neck uptake is absent but the patient is thyrotoxic (ectopic thyroid tissue concentrates iodine in the pelvis).

Related Pages

• Tracer: Radioiodine (I-123/I-131); diseases: Hyperthyroidism, Thyroid nodule.

Figure / Diagram Suggestions

• A low-uptake vs Graves diffuse-uptake teaching pair.
• A triphasic thyroiditis timeline (thyrotoxic → hypothyroid → recovery).
• A Doppler "low-flow destructive" vs "thyroid inferno" comparison.

Self-Check