A 40-year-old man presents with thyroid swelling and lung opacities. What is the most probable diagnosis?

In the context of a 40-year-old man presenting with thyroid swelling and lung opacities, the most probable diagnosis is follicular carcinoma. This form of thyroid cancer can present with local symptoms such as a palpable thyroid mass, and the potential for distant metastasis, particularly to the lungs, is a characteristic feature.

Follicular carcinoma is known for its tendency to invade blood vessels and spread hematogenously, often resulting in lung nodules or opacities. This capacity for vascular invasion distinguishes it from other types of thyroid pathology. While papillary carcinoma is more common and typically presents with lymphatic spread rather than distant metastases, lung opacities would be less characteristic of this type. Anaplastic carcinoma, while aggressive and associated with swelling, typically presents in an older demographic and is not often characterized by distant lung opacities at the onset. Thyroiditis, on the other hand, usually presents with inflammation and is not typically associated with malignant features such as lung metastases.

Thus, the presence of thyroid swelling coupled with lung opacities aligns more closely with the metastatic spread associated with follicular carcinoma, making it the most likely diagnosis in this scenario.