From the question stem, the patient most likely suffers from Hashimoto's thyroiditis. In Hashimoto thyroiditis, autoimmune attack on the thyroid gland leads to lymphocytic infiltration, the formation of several germinal centers, fibrosis, and metaplasia (change from normal cuboidal cells to Hurthle cells). Multinucleate giant cell formation in the thyroid is more consistent with subacute thyroiditis.
Hashimoto thyroiditis results when there is recognition of thyroid self-antigen. This leads to the activation of T-lymphocytes as well as the activation of B-lymphocytes. The result is the release of cytokines and auto-antibodies that attack the thyroid gland and mediate damage. CD8 T-lymphocyte cell killing also occurs. Combined, this leads to hypothyroidism and its typical clinical manifestations (weight gain, cold intolerance, constipation, thin nails, etc).
Bindra et al. outline the diagnosis and treatment of various types of thyroiditis, of which Hashimoto's is the most common. Other types include subacute granulomatous thyroiditis, postpartum thyroiditis, subacute lymphocytic thyroiditis, and drug-induced thyroiditis (caused by amiodarone, interferon-alfa, interleukin-2, or lithium).
Paknys et al. discuss the pathogenesis of Hashimoto thyroiditis. The researchers note that "serum antibodies reacting with thyroglobulin and thyroid peroxidase" are often found in patients with Hashimoto thyroiditis. They state that the resulting hypothyroidism seen in Hashimoto thyroiditis is the presentation of cell-mediated immunity leading to destruction of thyroid cells.
Illustration A shows a biopsy of Hashimoto's showing a dense infiltrate of lymphocytes with germinal center formation. Cells with pink granular cytoplasm are referred to as Hurthle cells, which is a metaplastic change.
Answers 2-5: These histologic findings are seen in Hashimoto's thyroiditis.
Bindra A, Braunstein GD. Thyroiditis. Am Fam Physician. 2006 May 15;73(10):1769-76.
PMID:16734054 (Link to Abstract)
Paknys G, Kondrotas AJ, Kevelaitis E. Risk factors and pathogenesis of Hashimoto's thyroiditis. Medicina (Kaunas). 2009;45(7)
PMID:19667753 (Link to Abstract)