Thyroid Pharmacology

Correct Answer

B — Sorafenib

Rationale

Sorafenib is a multi-kinase inhibitor targeting vascular endothelial growth factor receptor, platelet-derived growth factor receptor, and RAF kinases, approved for radioiodine-refractory differentiated thyroid cancer. Lenvatinib is the other approved multi-kinase inhibitor in this indication. Selpercatinib is a selective RET inhibitor — a distinct subclass. Levothyroxine is a thyroid hormone replacement. Methimazole is a thionamide antithyroid drug.

Correct Answer

D — Selpercatinib

Rationale

Selpercatinib is a selective RET inhibitor approved for RET-mutant medullary thyroid cancer and RET-fusion-positive differentiated thyroid cancer. Pralsetinib is the other selective RET inhibitor in this class. Vandetanib and cabozantinib are multi-kinase inhibitors that target RET among other kinases but are classified as multi-kinase inhibitors, not selective RET inhibitors. Sorafenib and lenvatinib are multi-kinase inhibitors used in radioiodine-refractory differentiated thyroid cancer and do not preferentially target RET.

Correct Answer

A — Dabrafenib

Rationale

Dabrafenib is a BRAF inhibitor used in combination with trametinib (a MEK inhibitor) for BRAF V600E-mutant anaplastic thyroid cancer, receiving approval after demonstrating approximately 69% response rates in the ROAR basket trial. Trametinib is its combination partner but is classified as a MEK inhibitor, not a BRAF inhibitor. Selpercatinib is a selective RET inhibitor. Cabozantinib is a multi-kinase inhibitor approved for medullary thyroid cancer.

Correct Answer

C — Thyrotropin alfa

Rationale

Thyrotropin alfa is recombinant human thyroid-stimulating hormone administered as 0.9 mg intramuscularly on two consecutive days before radioiodine scanning or ablation. It achieves the thyroid-stimulating hormone elevation above 30 mIU/L needed to maximize sodium-iodide symporter expression in remnant and metastatic tissue, while allowing the patient to remain euthyroid on continued levothyroxine therapy. Levothyroxine and liothyronine are thyroid hormone preparations that must be withdrawn to achieve this thyroid-stimulating hormone rise by the alternative method. Methimazole is a thionamide antithyroid drug.

Correct Answer

B — Vandetanib

Rationale

Vandetanib is a multi-kinase inhibitor targeting RET kinase and vascular endothelial growth factor receptor, approved for progressive medullary thyroid cancer. Cabozantinib is the other multi-kinase inhibitor in this indication. Selpercatinib is classified as a selective RET inhibitor — a distinct subclass with better tolerability than multi-kinase inhibitors. Dabrafenib is a BRAF inhibitor used in anaplastic thyroid cancer. Levothyroxine is a thyroid hormone replacement.

Correct Answer

A — Trametinib

Rationale

Trametinib is a MEK inhibitor combined with dabrafenib (a BRAF inhibitor) for BRAF V600E-mutant anaplastic thyroid cancer. MEK is a kinase downstream of BRAF in the mitogen-activated protein kinase signaling pathway; simultaneous inhibition of both nodes reduces the likelihood of resistance that emerges when only one is targeted. Dabrafenib is the BRAF inhibitor partner in this combination. Selpercatinib is a selective RET inhibitor. Sorafenib is a multi-kinase inhibitor for radioiodine-refractory differentiated thyroid cancer.

Correct Answer

C — Thyroid-stimulating hormone upregulates sodium-iodide symporter expression in remnant and metastatic thyroid tissue, maximizing radioiodine uptake

Rationale

The sodium-iodide symporter is the transporter that concentrates iodide — and radioiodine — within thyroid follicular cells and differentiated thyroid cancer metastases. Thyroid-stimulating hormone is the primary regulator of sodium-iodide symporter gene expression; when thyroid-stimulating hormone is elevated above 30 mIU/L, symporter expression increases substantially, maximizing radioiodine uptake per gram of target tissue and improving ablative efficacy. Without this stimulation, residual thyroid cells and metastases have low symporter expression and take up insufficient radioiodine for effective treatment. Thyroid-stimulating hormone does not alter iodine-131 decay physics or meaningfully change thyroid blood flow at the concentrations used clinically.

Correct Answer

B — Excess thyroid hormone activates thyroid hormone receptor alpha-1 in the heart, increasing heart rate, left ventricular mass, and atrial fibrillation risk

Rationale

Thyroid hormone receptor alpha-1 is the predominant thyroid hormone receptor isoform in the heart. When activated by excess triiodothyronine — whether from supraphysiological levothyroxine doses or from endogenous hyperthyroidism — it drives increased heart rate, left ventricular mass, and cardiac contractility, while also shortening atrial refractory periods and increasing susceptibility to atrial fibrillation. Thyroid hormone receptor beta-1 predominates in the liver rather than the heart and regulates cholesterol metabolism. Thyroid hormone acts through nuclear receptors, not through direct beta-adrenergic receptor activation, though the downstream cardiovascular effects overlap with sympathomimetic stimulation.

Correct Answer

D — Excess thyroid hormone activates osteoclasts and accelerates bone resorption, reducing cortical bone mineral density over years of exposure

Rationale

Thyroid hormone receptor alpha-1 is expressed in bone as well as the heart. Excess thyroid hormone acting at this receptor activates osteoclasts, accelerating bone resorption and uncoupling the normal balance between resorption and formation. The effect is predominantly on cortical bone and accumulates over years of sustained suppression below 0.1 mIU/L. Postmenopausal women are at greatest risk because estrogen deficiency independently accelerates cortical bone loss, and the two effects are additive. Bone mineral density screening and consideration of antiresorptive therapy are recommended for this population on sustained aggressive suppression. Thyroid hormone does not suppress parathyroid hormone, compete for vitamin D receptors, or act primarily through thyroid-stimulating hormone receptor pathways in bone.

Correct Answer

A — Amiodarone inhibits type 1 deiodinase, producing a predictable pharmacological pattern that represents drug effect, not primary thyroid disease requiring treatment

Rationale

In the first three months of amiodarone therapy, type 1 deiodinase inhibition produces a characteristic and expected pharmacological pattern: elevated free thyroxine (reduced peripheral conversion), low triiodothyronine (reduced conversion), elevated reverse triiodothyronine (impaired clearance), and a transient mild thyroid-stimulating hormone rise as the pituitary adjusts to the new thyroxine-to-triiodothyronine ratio. This pattern is a drug effect and should not trigger antithyroid treatment. True amiodarone-induced thyroid disease — type 1 or type 2 thyrotoxicosis — produces a different biochemical picture with thyroid-stimulating hormone fully suppressed and clinical thyrotoxic features. The timeline here (three weeks, mildly elevated thyroid-stimulating hormone) is consistent with expected pharmacological adjustment.

Correct Answer

C — Destructive thyroiditis from direct amiodarone cytotoxicity releasing preformed hormone; glucocorticoids suppress the inflammatory process

Rationale

Absent or markedly reduced color Doppler vascularity is the key discriminating feature of type 2 amiodarone-induced thyrotoxicosis, distinguishing it from type 1. Type 2 results from direct cytotoxic effects of amiodarone on thyroid follicular cells, producing destructive thyroiditis that releases preformed hormone without new synthesis. Because there is no ongoing synthesis to block, thionamides are not effective. Glucocorticoids — typically prednisone 40 mg per day tapered over three months — suppress the inflammatory destructive process and are the treatment of choice. Type 1 (iodine-driven synthesis with increased vascularity) is treated with high-dose methimazole plus potassium perchlorate. Color Doppler vascularity is the primary clinical tool for distinguishing these two entities when mixed features are absent.

Correct Answer

B — The hyperthyroid phase is caused by destructive autoimmune thyroiditis releasing preformed hormone, with no new synthesis occurring for thionamides to block

Rationale

Postpartum thyroiditis follows a characteristic triphasic course triggered by postpartum immune reconstitution. The initial hyperthyroid phase results from autoimmune destructive thyroiditis — lymphocytic infiltration damages follicular cells and releases preformed thyroxine and triiodothyronine from colloid stores. Since no new hormone synthesis is occurring, thionamides — which work by inhibiting thyroid peroxidase and blocking organification — have no target and are pharmacologically ineffective. Beta-blockers provide symptomatic relief of adrenergic symptoms during this phase. The subsequent hypothyroid phase may require temporary levothyroxine. This same mechanism explains the ineffectiveness of thionamides in other forms of destructive thyroiditis, including type 2 amiodarone-induced thyrotoxicosis.

Correct Answer

D — The mother's antithyroid drug crossed the placenta and suppressed fetal thyroid function in utero; as the drug clears over 3–7 days, thyroid-stimulating hormone receptor antibodies drive thyrotoxicosis

Rationale

When the mother is on propylthiouracil or methimazole, the antithyroid drug crosses the placenta and suppresses fetal thyroid hormone synthesis in utero, masking the thyroid-stimulating hormone receptor antibody-driven stimulation that would otherwise produce fetal thyrotoxicosis. After birth, the maternal drug clears from the neonatal circulation over 3–7 days as the neonate metabolizes it. Once drug levels fall, the thyroid-stimulating hormone receptor antibodies — which persist in the neonatal circulation with an immunoglobulin G half-life of several weeks — are no longer antagonized and drive thyroid hormone overproduction. This explains why a normal newborn screening thyroid-stimulating hormone at 36 hours does not exclude neonatal Graves disease, and why at-risk neonates require thyroid function testing again at 7–10 days.

Correct Answer

A — Depleting the stable iodine pool reduces competition at the sodium-iodide symporter, allowing a greater fraction of administered radioiodine to be taken up by thyroid tissue

Rationale

The sodium-iodide symporter transports all iodide species — stable iodide from dietary sources and radioiodine (iodine-131) — without distinguishing between isotopes. When the stable iodine pool is large, stable iodide competes with iodine-131 at the symporter, reducing the fraction of the administered radioiodine dose that is captured by thyroid tissue and metastases. A low-iodine diet (below 50 micrograms of iodine per day for one to two weeks) depletes this competing pool, allowing a greater proportion of the administered radioiodine to be taken up per gram of target tissue. This improves both ablative efficacy and scan sensitivity. Low dietary iodine does not directly activate the Wolff-Chaikoff effect or alter thyroid-stimulating hormone feedback in a clinically meaningful way on this timescale.

Correct Answer

D — Administer methimazole combined with glucocorticoids, because both mechanisms may be simultaneously active and each drug targets a different pathophysiological process

Rationale

Mixed forms of amiodarone-induced thyrotoxicosis — with features of both iodine-driven synthesis and destructive thyroiditis — are common and can be difficult to distinguish by ultrasound alone. When the type is uncertain, combined therapy with methimazole (targeting any active thyroid peroxidase-mediated synthesis in type 1) plus glucocorticoids (suppressing the inflammatory destructive process of type 2) is used empirically. This approach ensures that whichever mechanism is operative — or both simultaneously — is pharmacologically addressed. Withholding treatment while awaiting repeat imaging is clinically inappropriate in active thyrotoxicosis, and monotherapy risks leaving one mechanism untreated in a mixed presentation.

Correct Answer

C — Propylthiouracil carries a risk of idiosyncratic fulminant hepatic necrosis that makes its use unacceptable in neonates

Rationale

Propylthiouracil carries a black box warning for idiosyncratic fulminant hepatic necrosis — a risk that led to liver failure, liver transplantation, and death primarily in children and young patients. This hepatotoxicity risk makes propylthiouracil unacceptable for neonatal use. Methimazole is the preferred antithyroid agent in neonates, dosed at 0.2–0.5 mg per kilogram per day divided every 8 hours, with close thyroid function monitoring. Propranolol is co-prescribed for tachycardia control while antithyroid therapy establishes biochemical control. The course is self-limited as maternal thyroid-stimulating hormone receptor antibody titers decline over 3–6 months.

Correct Answer

B — Anti-thyroid peroxidase antibody positivity indicates active autoimmune thyroid destruction that may be sufficient to permanently deplete functional thyroid follicular cell mass

Rationale

Anti-thyroid peroxidase antibodies are markers of ongoing autoimmune thyroid inflammation — the same immune process responsible for Hashimoto thyroiditis. Their presence before or during postpartum thyroiditis indicates that the underlying autoimmune destruction is not simply a transient postpartum immune reconstitution event but reflects a sustained thyroid-directed immune response. When the follicular cell loss from this process exceeds the regenerative capacity of the remaining thyroid tissue, permanent hypothyroidism results. Approximately 25–30% of women with postpartum thyroiditis and positive anti-thyroid peroxidase antibodies develop permanent hypothyroidism requiring long-term levothyroxine replacement, compared with a much lower rate in antibody-negative women. Anti-thyroid peroxidase antibodies serve as immune markers of ongoing destruction rather than direct enzymatic inhibitors or thyroid-stimulating hormone receptor antagonists.

Correct Answer

A — Recombinant human thyroid-stimulating hormone achieves the required thyroid-stimulating hormone rise while the patient remains euthyroid on levothyroxine, avoiding the hypothyroid symptom burden of withdrawal

Rationale

The fundamental pharmacological advantage of recombinant human thyroid-stimulating hormone (thyrotropin alfa) is that it provides exogenous thyroid-stimulating hormone stimulation without requiring the patient to become hypothyroid. Thyroid hormone withdrawal achieves the same thyroid-stimulating hormone elevation but does so by withholding levothyroxine for four weeks, producing sustained hypothyroidism with fatigue, cognitive impairment, and reduced quality of life. For low-to-intermediate-risk patients, ablation outcomes with recombinant human thyroid-stimulating hormone are equivalent to withdrawal while preserving euthyroidism throughout. Withdrawal remains reserved for high-risk patients with known metastases requiring accurate dosimetry, where the patient's own thyroid-stimulating hormone elevation during hypothyroidism reflects true pharmacokinetics. Recombinant human thyroid-stimulating hormone does not produce higher peak thyroid-stimulating hormone levels than withdrawal, and does not reduce whole-body radiation exposure through the mechanism described.