Musculoskeletal Strain: Given the patient's recent history of intense physical labor due to the tornado and the description of the pain as a dull ache without numbness or tingling, a musculoskeletal strain is the most plausible diagnosis. The patient's rating of the pain as 7/8/10 suggests a significant level of discomfort that is consistent with overuse or strain injuries.

Fibromyalgia: The patient's history of hypothyroidism and myalgia (muscle pain) could suggest fibromyalgia, especially if the pain is widespread and not solely localized to the mid back. Fibromyalgia often presents with diffuse musculoskeletal pain and can be exacerbated by physical activity.
Osteoarthritis: Although the patient's age and symptoms could suggest osteoarthritis, the acute onset related to increased physical activity leans more towards a strain. However, osteoarthritis of the spine (spondylosis) could be a contributing factor, especially if there's a pre-existing condition.
Disc Herniation or Degenerative Disc Disease: The lack of numbness, tingling, or radiating pain makes this less likely, but it cannot be ruled out without further investigation, such as imaging studies.

Spinal Fracture: Especially in the context of intense physical labor and potential osteoporosis (which could be related to hypothyroidism if not well-managed), a spinal fracture must be considered. This would be particularly concerning if there was a traumatic event or significant strain.
Cancer (Metastatic or Primary): Although less common, back pain can be a presenting symptom of cancer, including metastases to the spine or primary spinal tumors. A thorough history and examination, potentially followed by imaging, are crucial to rule out such serious conditions.
Infection (Osteomyelitis, Discitis): Infections of the spine are rare but serious conditions that require prompt diagnosis and treatment. Risk factors might include recent trauma, immunocompromised state, or a history of intravenous drug use.

Spondyloarthritis: Conditions like ankylosing spondylitis could present with back pain, but they are less common and typically have a more chronic course with additional symptoms such as morning stiffness and improvement with activity.
Referred Pain from Visceral Sources: Although rare, mid back pain can sometimes be referred from visceral sources, such as pancreatic or gallbladder disease. The absence of other symptoms (e.g., jaundice, abdominal pain) makes this less likely, but it should be considered if other diagnoses are ruled out.