Patient description: A 16-year-old girl came to the clinic complaining of weight gain and abnormal menstrual cycles. Her BMI was 24, but she has gained 5 kg in the past few weeks. She gets tired more frequently and does not have energy to go dancing with her friends. Her lab results were remarkable for high TSH levels (15 mU/L) and low free T4 levels (0.18 ng/dl). Her anti-TPO levels were extremely high (136 IU/ml). She was diagnosed with Hashimoto disease. She does not smoke and she is not sexually active.

Clinical trial eligibility criteria: eligible ages (years): 18.0-999.0,  Thyroid Cancer Participants must have histologically or cytologically confirmed diagnosis of one of the following differentiated thyroid cancer (DTC) subtypes: 1. Papillary thyroid cancer (PTC) Follicular variant Variants (including but not limited to tall cell, columnar cell, cribriform-morular, solid, oxyphil, Warthin's-like, trabecular, tumor with nodular fasciitis-like stroma, Hürthle cell variant of papillary carcinoma, poorly differentiated) 2. Follicular thyroid cancer (FTC) Hurthle cell Clear cell Insular 2. Measurable disease meeting the following and confirmed by central radiographic review: 1. At least 1 lesion of greater than or equal to (>=)1.0 centimeter (cm) in the longest diameter for a non-lymph node or >=1.5 cm in the short-axis diameter for a lymph node that is serially measurable according to Response Evaluation in Solid Tumors (RECIST 1.1) using computed tomography/magnetic resonance imaging (CT/MRI). If there is only 1 target lesion and it is a non-lymph node, it should have a longest diameter of >=1.5 cm. 2. Lesions that have had external beam radiotherapy (EBRT) or locoregional therapies such as radiofrequency (RF) ablation must show evidence of progressive disease based on 1.1 to be deemed a target lesion. 3. Participants must show evidence of disease progression within 12 months (an additional month will be allowed to accommodate actual dates of performance of screening scans, that is, within less than or equal to <=13 months) prior to signing informed consent, according to 1.1 assessed and confirmed by central radiographic review of CT and/or MRI scans. 4. Participants must be Iodine-131 refractory/resistant as defined by at least one of the following: 1. One or more measurable lesions that do not demonstrate iodine uptake on any radioiodine scan. 2. One or more measurable lesions that have progressed according to 1.1 within 12 months (an additional month will be allowed to accommodate actual dates of performance of screening scans, ie, within ≤13 months) after Iodine-131 therapy, despite demonstration of radioiodine avidity at the time of that treatment by pre or posttreatment scanning. These participants must not be eligible for possible curative surgery. 3. Cumulative activity of Iodine-131 of greater than (>) 600 millicurie (mCi) or 22 gigabecquerels (GBq), with the last dose administered at least 6 months prior to study entry. 5. Participants with known brain metastases who have completed whole brain radiotherapy, stereotactic radiosurgery or complete surgical resection, will be eligible if they have remained clinically stable, asymptomatic, and off steroids for one month. 6. Participants must be receiving thyroxine suppression therapy and thyroid stimulating hormone (TSH) should not be elevated (TSH should be <=5.50 micro-international units per liter [mcIU/ML]). When tolerated by the participant, thyroxine dose should be changed to achieve TSH suppression (TSH <0.50 mcIU/ML) and this dose may be changed concurrently upon starting study drug treatment. 7. All chemotherapy or radiation-related toxicities must have resolved to Grade <2 severity per Common Terminology for Adverse Events (CTCAE v4.03), except alopecia and infertility. 8. Participants must have an Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1, or 2. 9. Adequately controlled blood pressure (BP) with or without antihypertensive medications, defined as BP ≤150/90 mmHg at Screening and no change in antihypertensive medications within 1 week prior to Cycle 1/Day 1. 10. Adequate renal function defined as calculated creatinine clearance >=30 mL/min per the Cockcroft and Gault formula. 11. Adequate bone marrow function: 1. Absolute neutrophil count (ANC) >=1500/mm^3 (>=1.5*10^3/uL) 2. Platelets >=100,000/mm^3 (>=100*10^9/L) 3. Hemoglobin >=9.0 g/dL 12. Adequate blood coagulation function as evidenced by an International Normalized Ratio (INR) <=1.5. 13. Adequate liver function: 1. Bilirubin <=1.5*upper limit of normal (ULN) except for unconjugated hyperbilirubinemia or Gilbert's syndrome. 2. Alkaline phosphatase, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) <=3*ULN (<=5*ULN if participant has liver metastases). If alkaline phosphatase is >3*ULN (in absence of liver metastases) or >5*ULN (in presence of liver metastases) AND the participant also is known to have bone metastases, the liver-specific alkaline phosphatase must be separated from the total and used to assess the liver function instead of total alkaline phosphatase. 14. Males or females age >=18 years at the time of informed consent. 15. Females must not be lactating or pregnant at Screening or Baseline (as documented by a negative beta-human chorionic gonadotropin [B-hCG] test with a minimum sensitivity of 25 IU/L or equivalent units of B-hCG. A separate baseline assessment is required if a negative screening pregnancy test was obtained more than 72 hours before the first dose of study drug. 16. All females will be considered to be of childbearing potential unless they are postmenopausal (amenorrheic for at least 12 consecutive months, in the appropriate age group and without other known or suspected cause) or have been sterilized surgically (ie, bilateral tubal ligation, total hysterectomy or bilateral oophorectomy, all with surgery at least 1 month before dosing). 17. Females of childbearing potential should avoid becoming pregnant and use highly effective contraception while on treatment with lenvatinib and for at least 1 month after finishing treatment. Females of childbearing potential must not have had unprotected sexual intercourse within 30 days before study entry and must agree to use a highly effective method of contraception (example, total abstinence, an intrauterine device, a contraceptive implant, an oral contraceptive, or have a vasectomized partner with confirmed azoospermia) throughout the entire study period and for 30 days after study drug discontinuation. Females who are using hormonal contraceptives must have been on a stable dose of the same hormonal contraceptive product for at least 4 weeks before dosing and must continue to use the same contraceptive during the study and for 30 days after study drug discontinuation. Women using oral hormonal contraceptives should add a barrier method. 18. Participants must voluntarily agree to provide written informed consent. 19. Participants must be willing and able to comply with all aspects of the protocol  Anaplastic or medullary carcinoma of the thyroid. 2. Diagnosed with meningeal carcinomatosis. 3. Two or more prior vascular endothelial growth factor (VEGF)/vascular endothelial growth factor receptor (VEGFR)-targeted therapies or any ongoing treatment for RR-DTC other than TSH-suppressive thyroid hormone therapy. 4. Prior treatment with lenvatinib. 5. Participants who have received any anticancer treatment within 21 days or any investigational agent within 30 days (or 5 half-lives) prior to the first dose of study drug and should have recovered from any toxicity related to previous anticancer treatment. This does not apply to the use of TSH suppressive thyroid hormone therapy. 6. Major surgery (example, laparotomy, thoracotomy or joint replacement) within 3 weeks prior to randomization or elective surgery scheduled to be performed during the study. 7. Participants having >1+ proteinuria on urine dipstick testing will undergo 24-hour urine collection for quantitative assessment of proteinuria. Participants with urine protein >=1 g/24 h will be ineligible. 8. Gastrointestinal malabsorption or any other condition that in the opinion of the investigator might affect the absorption of study drug. 9. Significant cardiovascular impairment: history of congestive heart failure greater than New York Heart Association (NYHA) Class II, unstable angina, myocardial infarction or cerebral vascular accident within 6 months of the first dose of study drug, or cardiac arrhythmia associated with hemodynamic instability. 10. Prolongation of corrected QT interval (QTc) to >480 ms as demonstrated by a repeated electrocardiogram (ECG) or a clinically significant ECG abnormality, including a marked prolonged QT/QTc interval (example, a repeated demonstration of a QTc interval >500 ms). 11. Active hemoptysis (bright red blood of at least 0.5 teaspoon) within 3 weeks prior to the first dose of study drug. 12. Active infection (any infection requiring treatment). 13. Active malignancy (except for DTC or definitively treated melanoma in-situ, basal or squamous cell carcinoma of the skin, or carcinoma in-situ of the cervix) within the past 24 months. 14. Bleeding or thrombotic disorders. 15. Known intolerance to study drug (or any of the excipients). 16. Any medical or other condition that in the opinion of the investigator(s) would preclude the participant's participation in a clinical study. 17. Females who are pregnant or breastfeeding