Thyroid Gland Anatomy and Development

• Thyroid gland development commences at 5 weeks gestation.
• The thyroglossal tract is a trail of epithelial cells that extends from superior to inferior, eventually solidifying and atrophying.
• By 7 weeks gestation, the thyroid divides into two lobes connected by an isthmus, and the thyroid cartilage is formed.
• Typical thyroid size is 15-20 grams, with each lobe measuring approximately 40-60mm in length and 13-18mm in the anterior-posterior (AP) dimension; the isthmus is 4-6mm thick.
• The thyroid has 4 arteries (superior and inferior thyroid arteries) and 3 veins (thyroid plexus on the anterior surface); peak velocity in major arteries is 20-40 cm/sec.
• Anatomical landmarks include the CCA and jugular vein laterally, longus colli muscle posteriorly, trachea medially, and strap muscles anterolaterally.
• Anatomic variants include aberrant configurations, heterotopic locations, thyroglossal duct, and athyrosis.
• The thyroid maintains body metabolism, physical and mental growth, lipolysis, and fatty acid metabolism, thereby lowering blood serum cholesterol.
• Thyroid hormones include Triiodothyronine (T3) and thyroxine (T4).
• T3 and T4 synthesis depends on the availability of iodine.
• T3 has a greater metabolic effect.
• Calcitonin (produced by C cells) lowers plasma calcium by inhibiting its release from bones.
• The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary to produce thyroid-stimulating hormone (TSH).
• Increased hormone production, circulation, and metabolism inhibit TRH and TSH release in a negative feedback loop.
• Hyperthyroidism is the excessive release of thyroid hormones.
• Hypothyroidism is a hormone deficiency.
• Euthyroid indicates normal lab values.

Thyroid Cysts

• True cysts (developmental): thyroglossal duct cyst (midline) and brachial cleft cyst (lateral to carotids).
• Complex cysts result from hemorrhage or degeneration of nodules; papillary carcinomas require FNA.

Nontoxic Goiter

• Enlarged gland without nodularity or functional disturbance.
• The gland doubles in size and weighs a few hundred grams.
• Features associated with increased malignancy: hypoechogenicity, microcalcifications, increased vascularity, irregular borders.

Hyperthyroidism/Thyrotoxicosis

• Can be caused by Graves' disease (autoimmune).
• The thyroid is normal or enlarged, and hypervascular ("thyroid inferno").

Hypothyroidism

• The most common thyroid function disorder.
• The most common cause is iodine insufficiency, but can also be caused by pituitary or hypothalamic disease.
• Hypothyroidism can be caused by lymphatic thyroiditis (autoimmune) also known as Hashimoto's.
• Sonographic appearance includes coarse texture, multiple ill-defined hypoechoic areas separated by thick fibrous strands, diffuse abnormality.
• The best indication of diffuse enlargement is an isthmus measurement greater than 1cm AP, and hypervascularity.

Thyroid Pathology During Pregnancy

• The second most common endocrinopathy in women of reproductive age.
• Physiological changes include increased TBG and HCG.
• Partial inhibition of the pituitary leads to a transient decrease in TSH between 8-14 weeks gestation.
• There is reduced plasma iodine, increased thyroid size, postpartum thyroiditis (PPT), decreased echogenicity, and diffuse enlargement.

Thyroid Carcinoma

• Papillary carcinoma is the most common and least aggressive.
• Hurthle cell carcinoma is grouped with follicular thyroid cancers but is more aggressive.
• Anaplastic carcinoma and lymphoma typically occur in individuals 60-65 years old and are aggressive with widespread mets.