1. What is the likely diagnosis?

Primary hypothyroidism

2. What are the most likely causes?

• Autoimmune thyroid disease (Hashimoto's thyroiditis)
• Radioactive iodine therapy for hyperthyroidism
• Thyroidectomy
• External beam radiotherapy

3. What additional aspects of the history and physical examination could provide relevant information to help in the diagnosis?

• Family history or the presence in the patient of other autoimmune endocrine disease
• History of radioactive iodine therapy
• History of thyroidectomy, history of neck radiation for lymphoma or head and neck cancer
• Symptoms suggestive of coronary artery disease which would change therapy

4. What additional tests would help confirm the diagnosis?

None. Thyroid autoantibodies are arguably insensitive and the cost does not justify the benefit.

5. What are the treatment options?

Thyroid hormone replacement therapy with non-generic l-thyroxine or a generic form that has undergone rigorous quality controlled supervision of the manufacturing process.

6. What are some of the cardiac risk factors that are present in this patient? How does that affect therapy?

• Moderate obesity
• Hhypertension
• Hypercholesterolemia 

Since she is of middle age with risk factors for coronary arterial disease, it would be wise to start with a low dose of thyroid hormone and raise the dose slowly.