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Summary
Thyroid cancer is the most common endocrine malignancy, with approximately 44,000 new diagnoses annually in the United States. While most differentiated thyroid cancers are curable with surgery and radioiodine (RAI) therapy, a meaningful subset develop RAI-refractory disease or harbor high-risk mutations that drive aggressive behavior. In 2026, clinical trials are focused on precision targeting of RET fusions, BRAF V600E alterations, and NTRK fusions, alongside novel redifferentiation strategies that restore RAI uptake in previously refractory tumors.
The Molecular Landscape of Thyroid Cancer
Thyroid cancers are classified into well-differentiated (papillary, follicular), poorly differentiated, anaplastic, and medullary subtypes — each with distinct genomic drivers. Papillary thyroid carcinoma (PTC), the most common form, is dominated by BRAF V600E mutations (~60%), RET/PTC rearrangements (~20%), and RAS mutations. Medullary thyroid carcinoma (MTC) is driven by RET point mutations, either germline (hereditary MEN2) or somatic (sporadic). Anaplastic thyroid carcinoma (ATC) is the most lethal, frequently harboring BRAF V600E alongside TP53, TERT promoter, and PI3K pathway alterations.
This molecular diversity has become clinically actionable. Molecular testing is now standard of care for any patient with advanced thyroid cancer, guiding selection among an expanding menu of targeted therapies. The identification of RET, BRAF, NTRK, and ALK alterations has transformed treatment algorithms that previously relied almost entirely on non-selective multikinase inhibitors (sorafenib, lenvatinib) with modest efficacy and significant toxicity.
RET-Targeted Therapies: Selpercatinib and Pralsetinib
The development of highly selective RET inhibitors — selpercatinib (Retevmo, Eli Lilly) and pralsetinib (Gavreto, Blueprint Medicines) — represents a paradigm shift in both MTC and RET fusion-positive differentiated thyroid cancer. Unlike earlier multikinase inhibitors, these agents are designed to spare VEGFR2, substantially reducing cardiovascular and hypertensive side effects while delivering higher RET inhibitory potency.
The LIBRETTO-001 trial (NCT03157128) established selpercatinib's activity across RET-altered cancers. In RET-mutant MTC previously treated with vandetanib or cabozantinib, selpercatinib achieved an overall response rate (ORR) of 69% with a median duration of response of 22 months. In treatment-naive RET-mutant MTC, ORR was 73%. The subsequent LIBRETTO-531 Phase 3 trial compared selpercatinib to cabozantinib or vandetanib in previously untreated RET-mutant MTC, demonstrating a significant PFS improvement (HR 0.28) — cementing selpercatinib as the preferred first-line standard. In 2026, LIBRETTO-531 extension cohorts are exploring outcomes after acquired RET resistance mutations (particularly RET G810 gatekeeper mutations) and testing combination with everolimus to overcome mTOR-driven resistance.
Pralsetinib, studied in the ARROW trial (NCT03037385), demonstrated an ORR of 71% in RET-mutant MTC and 89% in RET fusion-positive PTC with prior therapy. Both agents are now approved by the FDA, and head-to-head comparison trials are underway to differentiate their efficacy and tolerability profiles in specific RET mutation subtypes.
BRAF-Targeted Approaches in Papillary and Anaplastic Thyroid Cancer
BRAF V600E is the most common mutation in PTC and is present in nearly half of anaplastic thyroid cancers. The combination of dabrafenib (BRAF inhibitor) plus trametinib (MEK inhibitor) — already approved in BRAF V600E-mutant melanoma and NSCLC — has shown striking activity in ATC. A Phase 2 basket trial (NCT02034110) reported a 69% overall response rate in patients with BRAF V600E-mutant ATC, including complete responses — remarkable in a disease where median survival was historically under 5 months. FDA granted accelerated approval for this combination in ATC in 2018, the first systemic therapy ever approved for this disease.
In 2026, key trials are addressing resistance to BRAF/MEK inhibition in ATC, the optimal duration of therapy before surgery, and the combination of dabrafenib/trametinib with immunotherapy. The ATC-COMBO trial is evaluating spartalizumab (PD-1 inhibitor) added to dabrafenib/trametinib in BRAF V600E-mutant ATC, based on the hypothesis that MAP kinase inhibition increases tumor immunogenicity. Interim data suggest a possible improvement in complete response rate, though ATC's aggressive biology complicates trial completion.
Radioiodine-Refractory Disease: Redifferentiation Strategies
Approximately 5–15% of differentiated thyroid cancers become radioiodine (RAI)-refractory — defined by loss of iodine uptake on RAI scanning, failure to respond to prior RAI, or disease progression within 12 months of RAI therapy. Loss of RAI avidity is driven by dedifferentiation: downregulation of NIS (sodium-iodide symporter, encoded by SLC5A5) and other thyroid-specific genes required for iodine uptake and organification.
A key insight is that MAPK pathway activation (via BRAF V600E or RAS mutations) suppresses NIS expression. Inhibiting this pathway can restore NIS expression — a strategy called "redifferentiation." The MERIDIIAN trial demonstrated that vemurafenib (BRAF inhibitor) prior to RAI dosimetry increased RAI uptake in BRAF V600E-mutant RAI-refractory PTC, restoring clinically meaningful iodine incorporation in ~40% of patients. Building on this, the NEWSTART trial is evaluating selpercatinib-mediated redifferentiation in RET-altered RAI-refractory PTC, and the SEL-RET trial is assessing whether a defined "drug holiday" allows sufficient NIS re-expression for effective RAI therapy. Combination MEK inhibitor (cobimetinib) plus RAI redifferentiation is also in Phase 2 trials for RAS-mutant RAI-refractory disease.
NTRK Fusions and Tumor-Agnostic Approvals
NTRK1/2/3 fusions occur in approximately 2–5% of differentiated thyroid cancers and in a higher proportion (~15%) of pediatric thyroid cancers. Larotrectinib (Vitrakvi, Bayer/Loxo) and entrectinib (Rozlytrek, Genentech) received the first tumor-agnostic FDA approvals based on NTRK fusion positivity across any solid tumor. Larotrectinib demonstrated a 75% ORR in NTRK fusion-positive thyroid cancer in the NAVIGATE basket trial, with remarkably durable responses (ongoing at 2+ years in many patients).
Second-generation TRK inhibitors — selitrectinib and repotrectinib — are designed to overcome acquired resistance driven by NTRK kinase domain mutations (NTRK1 G595R, NTRK3 G623R). The TRIDENT-1 trial for repotrectinib is enrolling thyroid cancer patients with NTRK fusion-positive disease after prior TRK inhibitor exposure, with Phase 2 data showing a 40% ORR in this heavily pre-treated population. These results underscore that next-generation sequencing at diagnosis and at progression is essential in thyroid cancer to identify and re-address fusion driver alterations.
Lenvatinib Combinations and Novel Agents
Lenvatinib (Lenvima, Eisai/Merck), a multikinase inhibitor targeting VEGFR1-3, FGFR1-4, PDGFR, RET, and KIT, remains a cornerstone of RAI-refractory differentiated thyroid cancer treatment following the SELECT trial (ORR 65%, PFS 18.3 months vs. 3.6 months on placebo). In 2026, lenvatinib is being combined with pembrolizumab in the KEYNOTE-966 thyroid cancer expansion cohort and with everolimus (mTOR inhibitor) in RAI-refractory tumors with concurrent PI3K/AKT/mTOR pathway activation. The LENVA-COMBO trial is also testing lenvatinib sequencing versus combination with sorafenib as a rechallenge strategy in lenvatinib-progressed disease.
Key Takeaways
- Selpercatinib and pralsetinib have become the preferred first-line treatments for RET-mutant medullary and RET fusion-positive differentiated thyroid cancer, replacing older multikinase inhibitors.
- Dabrafenib plus trametinib achieves ~69% response rate in BRAF V600E-mutant anaplastic thyroid carcinoma and is the only FDA-approved systemic therapy for this aggressive disease.
- Redifferentiation strategies using targeted inhibitors (vemurafenib, selpercatinib, cobimetinib) before RAI therapy can restore iodine uptake in a meaningful subset of RAI-refractory differentiated thyroid cancer.
- NTRK fusion-positive thyroid cancers respond highly to larotrectinib and entrectinib; second-generation TRK inhibitors (selitrectinib, repotrectinib) address acquired resistance mutations.
- Comprehensive molecular profiling at diagnosis and at progression is now standard in advanced thyroid cancer and directly guides access to approved targeted therapies and clinical trial eligibility.