Normal histology	Histopathology	Pathological basis of disease	Clinical findings	
Tissue architecture:
•	The functional units of the thyroid gland are irregular spheroidal structures called thyroid follicles.
•	The thyroid gland is enveloped by an outer capsule of loose supporting tissue & an inner capsule of fibro-elastic tissue.
•	From the inner capsule, fine collagenous septa extend into the gland dividing the gland into lobules & conveying a rich blood supply together with lymphatics & nerves.

Cellular characteristics:
•	Thyroid follicles are lined by a simple cuboidal epithelium responsible for the synthesis of thyroid hormones.
•	Thyroid follicles are filled with a glycoprotein complex called thyroglobulin (colloid) which stores the thyroid hormones prior to secretion.
•	Parafollicular or ‘C’ cells are found in the thyroid gland interstitium as single cells scattered among follicular cells or as small clumps in the interfollicular spaces.
•	Parafollicular cells are identified by the immunoperoxidase stain with antibody to calcitonin.
•	Parafollicular cells synthesize the hormone calcitonin in direct response to raised blood calcium levels.	Abnormal histology:
•	Gland is mildly & symmetrically enlarged with an intact capsule.
•	Hypertrophy & hyperplasia of follicular epithelium, manifested by crowding of the columnar cells into irregular papillary folds.
•	Presence of peripheral scalloping colloid at interface of epithelium & colloid.
•	Follicular lining cells are very compacted & have tall columnar appearance.
•	Colloid is substantially decreased due to increased thyroxine secretion.
•	Interfollicular parenchyma contains hyperplastic lymphoid tissue & increased numbers of blood vessels. 	Incidence:
•	Peak incidence between ages of 20 & 30.
•	Women are affected up to 7 times more often than men.
•	Most common cause of endogenous hyperthyroidism.

Pathogenesis:
•	Autoimmune disorder produced by autoantibodies to TSH receptor, thyroglobulin & thyroid hormones.
•	Associated with HLA-B8 & DR3.

Associated with:
•	Hashimoto disease
•	Pernicious anemia
•	Rheumatoid arthritis	Clinical symptoms:
•	Hyperthyroidism resulting from hyperfunctional, diffuse enlargement of the thyroid.
•	Infiltrative ophthalmopathy with resultant exophthalmos.
•	Localized, infiltrative dermopathy called pretibial myxedema.

Signs of Hyperthyroidism:
•	Cardiac: tachycardia, palpitations, arrhythmais, atrial fibrillations.
•	Ocular changes: a wide, staring gaze & lid lag due to sympathetic overstimulation of the levator palpebrae superioris.
•	Neuromuscular: tremor, hyperactivity, anxiety, inability to concentrate & insomnia.
•	Skin: warm, moist & flushed.
•	Gastrointestinal: weight loss despite an increased appetite & hyperphagia; diarrhea due to increased gut motility.