Hyperthyroidism

Quick Answer

The radioiodine (or pertechnetate) uptake and scan distinguishes the causes of thyrotoxicosis by pattern: diffusely increased in Graves, focal/heterogeneous in toxic nodular disease, and low in thyroiditis or exogenous hormone. High-uptake causes are radioiodine-treatable; low-uptake thyrotoxicosis is not (nothing concentrates the iodine). In Graves, the therapeutic goal is a controlled hypothyroid endpoint — a randomized trial found fixed activity (typically 10–15 mCi) produced fewer treatment failures than titrating to euthyroidism.

Background & Pathophysiology

Thyrotoxicosis presents identically across causes but demands opposite management. Imaging answers the pivotal mechanistic question: is the gland making too much hormone (high uptake — Graves' TSH-receptor antibody stimulation, or autonomous nodules) or leaking/receiving it (low uptake — destructive thyroiditis, exogenous hormone, iodine load)?

Clinical & Laboratory

Suppressed TSH with elevated free T4/T3. Graves-specific features: orbitopathy, pretibial myxedema, thyroid bruit, and positive TRAb/TSI. Toxic nodular disease occurs in older patients with long-standing goiter. Check ESR/CRP (subacute thyroiditis) and thyroglobulin (low in exogenous).

Uptake Patterns

Doppler: hypervascular "thyroid inferno" in Graves vs low flow in destructive thyroiditis.

Radioiodine Therapy

• Graves: aim to render the patient hypothyroid (a controlled, easily treated endpoint), not euthyroid. A randomized trial found fixed activity (10–15 mCi) gave fewer treatment failures than dose-titration to euthyroidism; calculated dosing also performs well.
• Toxic nodular disease: calculated dosing is generally preferred — autonomous nodules concentrate iodine while the suppressed normal gland is relatively spared (lower post-treatment hypothyroidism than Graves).
• Pre-treatment: exclude pregnancy/lactation; avoid iodine load; consider orbitopathy risk (radioiodine can worsen active Graves ophthalmopathy — steroid cover or alternative therapy in moderate/severe eye disease, especially smokers).
• Safety: counsel on patient-release precautions.

Interpretation & Decision

High uptake vs low uptake decides everything. High-uptake Graves/nodular disease → I-131 or surgery or antithyroid drugs; low-uptake thyroiditis/exogenous → supportive only. Confirm no iodine load first (falsely low uptake).

Differential Diagnosis

Graves, toxic adenoma, toxic MNG, subacute/painless/postpartum thyroiditis, exogenous/factitious thyrotoxicosis, iodine-induced (Jod-Basedow), amiodarone types 1/2, struma ovarii, and rare TSH-secreting adenoma.

Reporting Checklist

• Report the RAIU value (timing/reference) and the scan pattern.
• Classify as radioiodine-treatable (high uptake) vs not (low uptake).
• For therapy, document written directive, activity, pregnancy exclusion, and orbitopathy risk.

Common Pitfalls

• Treating thyroiditis as Graves (wasted therapy).
• Not checking for an iodine load before uptake (falsely low uptake).
• Radioiodine in active moderate/severe Graves orbitopathy without steroid cover (can worsen eye disease).
• Radioiodine in children under ~5 (avoided; careful activity in ages 5–10).

Board Pearls

The uptake-and-scan pattern separates thyrotoxicosis into radioiodine-treatable and not: diffuse ↑ = Graves, focal ↑ with suppressed background = toxic nodular disease (both treatable); low/absent uptake = thyroiditis, exogenous hormone, or iodine load — the gland is leaking or loaded, not making, so there is no target for I-131. Always check for a recent iodine load first.

In Graves, aim for hypothyroidism (a controlled endpoint) — a randomized trial found fixed activity (10–15 mCi) gave fewer failures than titrating to euthyroidism. In toxic nodular disease, calculated dosing spares the suppressed normal gland (less post-treatment hypothyroidism).

Radioiodine can worsen active Graves orbitopathy — use steroid cover or an alternative in moderate/severe eye disease, especially smokers. Avoid I-131 in young children, pregnancy/lactation, and after an iodine load. Doppler "thyroid inferno" supports Graves when antibodies are equivocal.

Related Pages

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

Figure / Diagram Suggestions

• A three-pattern uptake plate (diffuse Graves / focal toxic nodule / low thyroiditis).
• A fixed vs calculated dosing decision aid (Graves vs nodular).

Self-Check