2. Morphology The functional unit of the thyroid gland is the follicle
It is composed of epithelial cells arranged as hollow vesicles of various shapes ranging in size from 0.02 to 0.3 mm in diameter
It is filled with a glycoprotein colloid called thyroglobulin
Groups of densely packed follicles are bound together by connective tissue septa to form lobules
Single layer of epithelial cells (red arrow)
Parafollicular cell (white arrow)
Connective tissue septum separating two lobules (green arrow)

3. Secretory cells of the thyroid gland are of two types:
Follicular cells, which produce the classical thyroid hormones, thyroxine and triiodothyronine
Parafollicular, or C cells, are located between the follicles and produce the polypeptide hormone calcitonin

4. Thyroid Hormones
The thyroid hormones are α-amino acid derivatives of tyrosine
Thyroxine & Triiodothyronine are exceptionally rich in iodine, which comprises more than half of their molecular weight
Thyroxine contains four atoms of iodine and is abbreviated as T4
Triiodothyronine, which has three atoms of iodine, is abbreviated as T3
Alpha amino acids having both the amine and the carboxylic acid groups attached to the first carbon

5. Feedback Regulation of Thyroid Hormone Secretion
Measurement of relative concentrations of TSH and thyroid hormones in the blood provide important information for diagnosing thyroid disease.
For instance, low blood concentrations of free T3 and T4 in the presence of elevated levels of TSH signal a primary defect in the thyroid gland
while high concentrations of free T3 and T4 accompanied by high concentrations of TSH reflect a defect in the pituitary or hypothalamus

6. Hypothyroidism Etiology of hypothyroidism
Hypothyroidism may be classified as
Primary
thyroid failure (most common)
Etiology
Hashimoto’s thyroiditis:
Radioactive iodine therapy for Graves’ disease
Subtotal thyroidectomy for Graves’ disease
Excessive iodide intake
Iodide deficiency
Inborn errors of thyroid hormone synthesis
Drugs (Lithium)
Removal of majority of both lobes leaving behind 4-5 grams (equivalent to the size of a normal thyroid gland) of thyroid tissue on one or both sides
The mechanism by which lithium inhibits thyroid hormone release is not well understood. In vitro, lithium decreases colloid droplet formation within thyroid follicular cells, a reflection of decreased pinocytosis of colloid from the follicular lumen

7. Peripheral resistance to the action of thyroid hormones
Secondary
due to pituitary TSH deficiency
Tertiary
due to hypothalamic deficiency of TRH
Peripheral resistance to the action of thyroid hormones

8. Graves’ Disease
Graves’ disease is the most common form of thyrotoxicosis
Females are involved about five times more commonly than males
Autoimmune disease of unknown cause
The syndrome consists of one or more of the following features:
thyrotoxicosis
goiter
ophthalmopathy
dermopathy
inflammation of the eyes and bulging eyes.

9. Treatment of Graves’ Disease
Although autoimmune mechanisms are responsible for the syndrome of Graves’ disease, management has been largely directed toward controlling the hyperthyroidism
Three good methods are available:
Antithyroid Drug therapy
inhibiting TPO-mediated iodination of thyroglobulin
Surgery
Partial or total thyroidectomy
and radioactive iodine therapy (131I)
destroys thyroid cells
A dangerous and short lived fission product. Iodine 131 is a radioisotope with a very short half-life of 8.02 days, making it highly radioactive