Hyperthyroidism: A High-Yield USMLE Review
Hyperthyroidism is a state of excess thyroid hormone action producing suppressed TSH with elevated free T4 and/or free T3. On the exam, the central task is confirming the biochemical pattern and then using radioactive iodine uptake (RAIU) to distinguish overproduction (Graves', toxic nodule, toxic multinodular goiter) from destruction or exogenous hormone (thyroiditis). Recognizing thyroid storm as a life-threatening emergency is essential.
Pathophysiology
Excess circulating thyroid hormone drives up metabolic rate and adrenergic tone, producing the hypermetabolic symptoms. In Graves' disease, thyroid-stimulating immunoglobulins (TSI) bind and activate the TSH receptor without feedback regulation, so the gland keeps producing hormone regardless of levels; because the TSH receptor is also expressed on orbital fibroblasts and adipocytes, autoimmune inflammation causes glycosaminoglycan deposition, orbital muscle edema/fibrosis, and increased orbital fat, yielding proptosis and impaired eye movement. In thyroiditis (subacute/de Quervain, postpartum, hashitoxicosis), inflammatory follicular destruction leaks stored hormone, causing transient hyperthyroidism without active synthesis.
Presentation
- Heat intolerance, palpitations, and unintentional weight loss (often with increased appetite)
- Tachycardia, fine tremor, anxiety, irritability, and insomnia
- Warm, moist skin; may have atrial fibrillation, especially in older patients
- Goiter (diffuse in Graves', nodular in toxic multinodular goiter)
- Exophthalmos/prominent eyes, lid lag, and diplopia — specific to Graves' ophthalmopathy
- Pretibial myxedema (dermopathy) — rare, seen in Graves' disease
Diagnosis
- TSH — the best screening test; a suppressed TSH signals hyperthyroidism due to the logarithmic TSH–free T4 relationship
- Free T4 (and free T3) — confirm and quantify; both elevated with suppressed TSH confirms hyperthyroidism
- Radioactive iodine uptake (RAIU) — the key differentiator: HIGH uptake indicates overproduction (Graves', toxic adenoma, toxic multinodular goiter); LOW uptake indicates destruction/leak or exogenous hormone (thyroiditis)
- TSI/TRAb (TSH receptor antibodies) — positive in Graves' disease; RAIU in Graves' shows diffusely increased uptake
- RAIU scan pattern helps localize cause: diffuse uptake (Graves'), single hot nodule (toxic adenoma), patchy hot and cold areas (toxic multinodular goiter)
Management
- Beta-blocker (propranolol preferred) for symptomatic control of tachycardia, tremor, and agitation; propranolol also blocks peripheral T4→T3 conversion
- Antithyroid drugs (methimazole, or PTU) to block new hormone synthesis — appropriate for overproduction causes such as Graves' and toxic nodular disease
- Radioactive iodine ablation is definitive therapy, but the patient should be rendered euthyroid first; RAI is contraindicated in pregnancy
- Thyroiditis-related hyperthyroidism is transient — treat symptoms with beta-blockers and do NOT use antithyroid drugs (they won't work since there is no overproduction)
- Pregnancy: PTU preferred in the first trimester (methimazole causes embryopathy), then switch to methimazole in the second/third trimester (PTU has higher hepatotoxicity risk); use the lowest effective dose
- Thyroid storm: beta-blocker first, then antithyroid drug (PTU preferred), then iodine given ~1 hour after the antithyroid drug, plus glucocorticoids and supportive care (cooling, IV fluids, treat precipitant)
High-yield
- Suppressed TSH + elevated free T4 = hyperthyroidism; TSH is the best screening test
- RAIU splits the differential: HIGH = overproduction (Graves', toxic nodule/TMG); LOW = destruction or exogenous hormone (thyroiditis)
- Graves' is the only cause of hyperthyroidism with ophthalmopathy (exophthalmos) and is driven by TSI activating the TSH receptor
- Hot (functioning) nodules are almost never malignant
- Thyroid storm triad: high fever (>40°C), tachycardia out of proportion to fever, and altered mental status — triggered by surgery, infection, iodine load, or abrupt antithyroid drug withdrawal
- In thyroid storm, PTU (not methimazole) also blocks T4→T3 conversion; give antithyroid drug before iodine
- Postpartum thyroiditis classically presents ~4 months postpartum with low RAIU; 20–30% develop permanent hypothyroidism
- Subacute (de Quervain) thyroiditis: painful, tender thyroid after viral URI with markedly elevated ESR and low RAIU
Pitfalls
- Do not treat thyroiditis (thyroiditis-related hyperthyroidism) with antithyroid drugs — the problem is hormone leak, not overproduction
- Beta-blocker alone controls symptoms but does not treat the underlying hyperthyroidism in overproduction causes
- Giving iodine before the antithyroid drug in thyroid storm provides substrate for more hormone synthesis — always give the antithyroid drug first
- Methimazole is teratogenic in the first trimester (aplasia cutis, choanal/esophageal atresia — "methimazole embryopathy"); use PTU periconception and early pregnancy
- Do not send a patient to radioactive iodine ablation before rendering them euthyroid, and never use RAI in pregnancy
- Don't order RAIU to characterize a nodule when TSH is normal — RAIU is for evaluating hyperthyroidism, not nodule cancer risk (use ultrasound/FNA)
Don't just memorize Hyperthyroidism — practice reasoning through it on branching cases where your decisions shape the patient.