Thyroid Disorders: High-Yield USMLE Review
Thyroid disease is best approached through the feedback axis: TSH is the exquisitely sensitive screening test, and its relationship to Free T4 tells you whether the problem is primary (thyroid) or secondary (pituitary). Once dysfunction is confirmed, the next question is mechanism — overproduction versus destruction for thyrotoxicosis, autoimmune versus central for hypothyroidism, and benign versus malignant for nodules. Master the testing sequence, the RAIU dichotomy, and the special situations (pregnancy, storm, myxedema coma) and most thyroid questions become straightforward.
Thyroid Function Testing & Framework
TSH is the best screening test for thyroid dysfunction because of the logarithmic relationship between TSH and Free T4 — a small change in T4 produces a large change in TSH, so TSH becomes abnormal before T4 is obviously abnormal, and a normal TSH essentially excludes primary thyroid disease. Order TSH first; if abnormal, add Free T4 to confirm and quantify, then follow the pattern. TSH↑ with T4↓ = primary hypothyroidism → check TPO antibodies. TSH↓ with T4↑ = hyperthyroidism → RAIU scan or TSI antibodies. Apply the primary-versus-secondary fork: a high tropic hormone with low end-organ hormone means primary gland failure, while a low tropic hormone with low end-organ hormone means secondary (central) failure. A low Free T4 with an inappropriately 'normal' or low TSH points to secondary hypothyroidism from pituitary disease.
Hyperthyroidism & Thyrotoxicosis
Suppressed TSH with elevated Free T4 confirms hyperthyroidism (heat intolerance, palpitations, tremor, weight loss, tachycardia). The radioactive iodine uptake (RAIU) scan differentiates the cause: HIGH uptake means overproduction (Graves' disease, toxic multinodular goiter, toxic adenoma), while LOW uptake means destruction or exogenous hormone (thyroiditis, medication). Graves' disease is unique as the only cause of hyperthyroidism with ophthalmopathy — exophthalmos results from autoimmune inflammation of retroorbital tissues — and it uniquely can be treated medically long-term with antithyroid drugs. Toxic multinodular goiter presents in older patients with a nodular goiter and patchy uptake (mixed hot and cold areas) and no eye findings. Treatment options include antithyroid drugs (methimazole), beta-blockers such as propranolol for symptomatic relief, radioactive iodine ablation (definitive, but render the patient euthyroid first), and thyroidectomy. A beta-blocker alone does not treat the underlying hyperthyroidism.
Thyroiditis
Thyroiditis causes a transient hyperthyroid phase (release of preformed stored hormone from gland destruction) followed by a hypothyroid phase. The key clue is a LOW RAIU — the gland is not overproducing hormone, distinguishing it from Graves' and toxic nodules, which have HIGH uptake. Postpartum thyroiditis classically appears a few months after delivery with palpitations and tremor, suppressed TSH, elevated Free T4, and low RAIU; manage with beta-blockers for symptoms since the hyperthyroid phase resolves, a hypothyroid phase may follow in 2–3 months, and 20–30% develop permanent hypothyroidism. Subacute thyroiditis features a painful thyroid. Do NOT treat thyroiditis with antithyroid drugs — they will not work because there is no overproduction. Hashimoto's can rarely cause a transient hyperthyroid phase ('hashitoxicosis').
Hypothyroidism
Primary hypothyroidism shows a markedly elevated TSH with low Free T4; the most common cause is Hashimoto's thyroiditis, confirmed by positive TPO antibodies. Symptoms include fatigue and cold intolerance. Secondary (central) hypothyroidism, as in Sheehan syndrome after postpartum hemorrhage, produces a low Free T4 with an inappropriately normal or low TSH — recognize this as a pituitary problem, not primary thyroid disease. Before starting levothyroxine in suspected hypopituitarism, check morning cortisol: starting thyroid hormone raises metabolic rate and can precipitate adrenal crisis if cortisol is deficient, so replace glucocorticoid first. Myxedema coma is the life-threatening extreme of hypothyroidism.
Thyroid Nodules & Cancer
Most thyroid nodules are benign. Use ultrasound features and TSH to determine which need fine-needle aspiration (FNA). Concerning ultrasound features include a hypoechoic nodule with microcalcifications. Hot (functioning) nodules are almost never malignant, but cold nodules — including cold nodules within a toxic multinodular goiter — may be malignant and warrant evaluation. FNA cytology is reported by the Bethesda system; Bethesda III (atypia of undetermined significance) carries a 10–30% malignancy risk and is managed with molecular testing or repeat FNA rather than immediate surgery. Medullary thyroid cancer is special: it does not take up iodine, secretes calcitonin, and may be hereditary — check for a RET mutation.
Special Situations: Pregnancy & Emergencies
Pregnancy changes thyroid management: TSH targets are lower, and hypothyroid patients need an increased levothyroxine dose. For a Graves' patient planning pregnancy, switch from methimazole to PTU before conception because methimazole is teratogenic in the first trimester ('methimazole embryopathy' — aplasia cutis, choanal atresia, esophageal atresia). PTU has more first-trimester safety data; after the first trimester, switch back to methimazole because PTU carries higher hepatotoxicity risk. Uncontrolled hyperthyroidism in pregnancy is dangerous (miscarriage, preeclampsia), so do not simply stop all antithyroid drugs. Radioactive iodine is contraindicated in pregnancy and requires waiting after ablation before conceiving. Thyroid storm and myxedema coma are emergencies demanding immediate recognition and aggressive treatment. Post-thyroidectomy patients can develop acute hypocalcemia (perioral tingling, Trousseau sign) requiring IV calcium gluconate.
High-yield
- TSH is the single best screening test for thyroid dysfunction due to the logarithmic TSH–Free T4 relationship; a normal TSH essentially excludes primary thyroid disease.
- RAIU dichotomy: HIGH uptake = overproduction (Graves', toxic nodule); LOW uptake = destruction or exogenous hormone (thyroiditis, medication).
- Exophthalmos/ophthalmopathy is specific to Graves' disease among causes of hyperthyroidism.
- Hashimoto's thyroiditis = most common primary hypothyroidism; confirm with positive TPO antibodies.
- Thyroiditis = transient hyperthyroid phase then hypothyroid phase; treat symptoms with beta-blockers, NOT antithyroid drugs.
- Hot nodules are almost never malignant; cold nodules may be malignant.
- Bethesda III (atypia of undetermined significance) = 10–30% malignancy risk → molecular testing or repeat FNA.
- Medullary thyroid cancer secretes calcitonin, does not take up iodine, and is associated with RET mutations.
- Pregnancy: PTU in the first trimester, lower TSH targets, higher levothyroxine requirement; radioactive iodine is contraindicated.
- In suspected central hypothyroidism/hypopituitarism, replace glucocorticoid before levothyroxine to avoid adrenal crisis.
Pitfalls
- Treating thyroiditis with antithyroid drugs — they do not work because there is no hormone overproduction.
- Starting levothyroxine before checking cortisol in hypopituitarism, precipitating adrenal crisis.
- Interpreting a low Free T4 with 'normal' TSH as reassuring rather than recognizing secondary hypothyroidism.
- Giving a beta-blocker alone and thinking the underlying hyperthyroidism has been treated.
- Continuing methimazole into the first trimester of pregnancy (methimazole embryopathy) or, conversely, stopping all antithyroid drugs and leaving hyperthyroidism uncontrolled.
- Proceeding to radioactive iodine ablation without first rendering the patient euthyroid and without evaluating cold nodules.
- Forgetting that cold nodules within a toxic multinodular goiter can still harbor malignancy while hot nodules essentially do not.
- Assuming Graves' and thyroiditis look the same on RAIU — Graves' has high uptake, thyroiditis has low uptake.
Turn this into reasoning you can use on exam day — practice Thyroid Disorders on branching cases where your decisions shape the patient.