Thyroid and Parathyroid Glands: Anatomy

• Study notes relating to the thyroid and parathyroid

Thyroid Development

• Thyroid development starts at 5 weeks gestation.
• By 7 weeks, the thyroid divides into two lobes connected by an isthmus, and the thyroid cartilage forms.
• The thyroglossal tract (duct) leaves a trace of epithelial cells that solidify and atrophy along a superior to inferior path.

Thyroid Anatomy

• The thyroid typically weighs 15-20 grams.
• Each lobe is about 40-60mm in length and 13-18mm in the anteroposterior (AP) dimension, with the isthmus measuring 4-6mm thick.
• The thyroid receives blood supply from 4 arteries (superior and inferior thyroid arteries) with peak velocities of 20-40 cm/sec, and drains via 3 veins (thyroid plexus on the anterior surface).
• Anatomical landmarks include the common carotid artery (CCA) and jugular vein (lateral), longus colli muscle (posterior), trachea (medial), and strap muscles (anterolateral border).
• Anatomic variants include aberrant configurations, heterotopic locations, thyroglossal ducts, and athyrosis.

Thyroid Physiology and Hormones

• The thyroid maintains body metabolism, physical and mental growth, and lipolysis and fatty acid metabolism, which lowers blood serum cholesterol.
• The thyroid produces triiodothyronine (T3), thyroxine (T4), and calcitonin (or thyrocalcitonin).
• T3 and T4 synthesis depends on the availability of iodine; T3 has a greater metabolic effect.
• Calcitonin (produced by C cells) lowers plasma calcium levels by inhibiting calcium release from bones.

Negative Feedback System

• The hypothalamus releases thyrotropin releasing hormone (TRH).
• The pituitary produces thyroid stimulating hormone (TSH).
• This hormone production leads to increased circulation and metabolism, which inhibits TRH and TSH release.

Thyroid Dysfunction

• Hyperthyroidism is an excessive release of thyroid hormones.
• Hypothyroidism is a hormone deficiency.
• Euthyroid refers to normal lab values.

Thyroid Cysts

• True cysts (developmental) include thyroglossal duct cysts (midline) and brachial cleft cysts (lateral to the carotids).
• Complex cysts may result from hemorrhage or degeneration of nodules, and papillary carcinomas may require fine needle aspiration (FNA).

Nontoxic Goiter

• Nontoxic goiter presents as an enlarged gland without nodularity or functional disturbance
• It may double in size and weigh a few hundred grams.
• Features associated with increased malignancy risk include hypoechogenicity, microcalcifications, increased vascularity, and irregular borders.

Hyperthyroidism Specifics

• Hyperthyroidism, also known as thyrotoxicosis (hormone leakage from a nonhyperactive gland) or Graves' disease (autoimmune), may involve a normal or enlarged gland with hypervascularity ("thyroid inferno").

Hypothyroidism Specifics

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

Thyroid Pathology in Pregnancy

• Thyroid pathology is the second most common endocrinopathy in women of reproductive age.
• Maternal physiological changes during pregnancy include increased thyroxine-binding globulin (TBG) and human chorionic gonadotropin (HCG).
• Partial pituitary inhibition leads to a transient decrease in TSH between 8-14 weeks gestation, with reduced plasma iodine and increased thyroid size.
• Postpartum thyroiditis (PPT) may occur, characterized by decreased echogenicity and diffuse enlargement.

Thyroid Carcinoma

• Papillary carcinoma is the most common and least aggressive type of thyroid cancer.
• Hurthle cell carcinoma is grouped with follicular thyroid cancers but is more aggressive.
• Anaplastic carcinoma and lymphoma typically occur in individuals aged 60-65 and are aggressive with widespread metastases.