A 52-year-old woman presents to her primary care physician for a routine check-up. She casually mentions that a thyroid nodule, which was previously evaluated and found to be benign, now feels "a bit firmer" than before. Two and a half years ago, she had an evaluation for the nodule, which measured 2.0 cm on ultrasound, categorized as low suspicion for malignancy (isoechoic, solid, without microcalcifications or irregular margins). A fine-needle aspiration biopsy (FNAB) at that time yielded benign cytology. On physical examination today, her vital signs are stable, including a blood pressure of 120/75 mmHg and a heart rate of 72 beats/min. A 2.2 cm firm, mobile nodule is palpable in the left upper pole of the thyroid. No cervical lymphadenopathy is noted. Laboratory studies show a thyroid-stimulating hormone (TSH) level of 2.8 mIU/L (reference range: 0.34 - 5.60 mIU/L) and a Free Thyroxine (FT4) level of 15.2 pmol/L (reference range: 10.0 - 20.0 pmol/L). Given her history and current presentation, what is the most appropriate diagnostic approach and the subsequent initial management strategy?

A 48-year-old woman presents to her primary care physician's office after self-discovering a persistent neck lump approximately one month ago. She describes it as painless and denies any associated dysphagia, hoarseness, or respiratory difficulties. Her medical history is notable only for a distant history of therapeutic radiation to the head and neck region during childhood for an unrelated condition. She takes no regular medications. On physical examination, a solitary, firm, 2.5-cm nodule is palpated in the left thyroid lobe. The nodule feels somewhat fixed to the underlying structures and shows limited upward movement with swallowing. No palpable cervical lymphadenopathy is noted. Given these findings, what is the most appropriate initial diagnostic step, and which characteristic observed during this step would most strongly indicate a need for surgical intervention for presumed malignancy?

A 45-year-old woman presents to the outpatient endocrinology clinic for her annual follow-up, one year after undergoing total thyroidectomy and comprehensive central and lateral neck dissection for medullary thyroid cancer. Her initial tumor was 2 cm, and pathology revealed 5 out of 20 dissected lymph nodes were positive for malignancy. The fasting serum calcitonin level before surgery was 250 ng/L (< 4 ng/L). Her postoperative course was uneventful, and she has been consistently taking levothyroxine 0.125 mg/d. Her most recent TSH level, obtained 8 weeks prior, was 1.5 mIU/L. On physical examination, there are no palpable neck masses or other remarkable findings. Her current calcitonin level is 180 ng/L, and her Carcinoembryonic Antigen (CEA) level is 15 µg/L (< 5 µg/L). Considering her clinical presentation and laboratory findings, what is the most probable cause for the persistently elevated calcitonin and the most appropriate next step in her diagnostic and therapeutic management?

A 55-year-old man presents to a specialized endocrine clinic complaining of persistent watery diarrhea, unexplained flushing episodes, and difficulty swallowing, which he attributes to a sensation of a mass in his throat. These symptoms have been progressing over several months. On physical examination, a firm, non-tender, solitary nodule is palpated in the right lobe of the thyroid, measuring approximately 3 cm. Additionally, multiple palpable, enlarged, and firm lymph nodes are noted in the ipsilateral cervical region. Initial laboratory tests performed by his primary care physician revealed significantly elevated serum calcitonin levels and an elevated carcinoembryonic antigen (CEA). Considering these findings, what is the most appropriate next diagnostic step to confirm the nature of the thyroid lesion and guide the immediate pre-operative assessment for this patient?

A 45-year-old male presents to the outpatient endocrinology clinic for follow-up of a thyroid nodule. A recent fine needle aspiration biopsy confirmed the diagnosis of medullary thyroid carcinoma. His initial serum calcitonin was significantly elevated at 300 pg/mL (normal range 0-10 pg/mL), and subsequent genetic testing identified a germline mutation in the RET proto-oncogene. Physical examination reveals a firm, mobile thyroid nodule with no palpable cervical lymphadenopathy. Blood pressure is 130/85 mmHg, heart rate 72 bpm, and he appears generally well, denying current symptoms of palpitations, headaches, or anxiety. Lab results include a white blood cell count of 7.5 x 10^9/L (normal range 4.0-11.0 x 10^9/L), hemoglobin of 145 g/L (normal range 130-170 g/L), platelet count of 250 x 10^9/L (normal range 150-450 x 10^9/L), and serum calcium of 2.3 mmol/L (normal range 2.1-2.5 mmol/L). His carcinoembryonic antigen (CEA) is 15 ng/mL (normal range 0-5 ng/mL). Given these findings, what critical diagnostic evaluation should precede definitive surgical management, and what is the primary surgical approach for this condition?

A 65-year-old woman presents to an outpatient endocrinology clinic for follow-up regarding her long-standing hypothyroidism, managed with levothyroxine due to Hashimoto's thyroiditis. She reports no new symptoms, specifically denying dysphagia, hoarseness, or sudden growth of the nodule. Her medical history is otherwise unremarkable, with no family history of thyroid cancer or prior radiation exposure to the head and neck. On physical examination, her vital signs are stable. The thyroid gland appears diffusely enlarged, and a firm, mobile 2-cm nodule is distinctively palpable in the lower pole of the right lobe. No cervical lymphadenopathy is noted. Considering these findings, what is the most appropriate initial diagnostic imaging study, and what subsequent management step would be indicated if suspicious features are identified?