Microanatomy of the thyroid gland

Question 1

Embryology of the thyroid

Answer 1

• Follicular cells and parafollicular (C) cells are derived from endoderm
• Follicular cells arrange in a circle around colloid (Tg) w/ fenestrated capillaries running through the CT of the follicles
• Follicular cells produce T3/T4, parafollicular cells produce calcitonin

Question 2

Process of T3/T4 production

Answer 2

• Tg is iodinated on the apical membrane by TPO, then internalized
• The endosome fuses w/ lysosome which cleaves the Tg into bits
• The endosome then fuses w/ the basolateral membrane and the BM to ender the capillaries
• T4 is made at a 20x higher number than T3

Question 3

Function of thyroid hormone

Answer 3

• Normal fetal growth an development
• Regulates metabolism of tissues: increases metabolism of protein/carbs/fats
• Increases absorption from intestines
• Increases heartbeat/catecholaminergic state
• Regulates mental activity

Question 4

Difference in appearance of follicles in hyper vs hypothyroid states

Answer 4

• In hyperthyroid state there is: increased rER, microvilli, mitoses, vascularity
• The cells are tall and there is reduced/absent colloid w/ intracellular colloid droplets
• These changes are because there is greatly increased resorption of the colloid in hyperthyroid states
• In hypothyroid states there is decreased rER, microvilli, vascularity, and no mitoses
• The cells are flat w/ no intracellular colloid deposits, and the extracellular colloid is markedly increased
• This is b/c in hypothyroid states there is greatly reduced resorption of the colloid

Question 5

Functions of calcitonin

Answer 5

• Made by parafollicular (C) cells, calcitonin is an antagonist to parathyroid hormone
• Both hormones act to maintain a normal calcium level
• Calcitonin lowers blood calcium by suppressing bone resorption and increasing rate of bone ossification
• Calcitonin release regulated directly by serum Ca levels

Question 6

Hypothyroidism

Answer 6

• During development leads to cretinism
• Other causes: hashimotos (autoimmune), iodine deficiency, aging, lack of TSH
• Sx: cold intolerance, dry skin, lethargy, weakness, weight gain, apathy, mental sluggishness, bradycardia, constipation

Question 7

Hyperthyroidism

Answer 7

• Causes: graves disease (autoimmune), subacute thyroiditis (inflammation following viral URI), TSH producing pituitary adenoma, exogenous T4
• Sx: heat intolerance, flushed skin, increased appetite, muscle wasting, weight loss, heart palpitations, tremor, SOB, restlessness, nervousness, bulging eyes (other eye involvements), hyperdefecation, tachycardia

Question 8

Goiter and thyroid follicle look alike

Answer 8

• Goiters are a globally enlarged thyroid gland, generally there are a few common causes
• Could be iodine deficiency (very rare in US), graves, subacute thyroiditis, or hashimotos
• Histologically, normal follicular cells look like active mammary gland of a lactating women

Question 9

Parathyroid histology

Answer 9

• 4 parathyroid glands, 2 on each side of the larynx just posterior to the thyroid
• Each one is surrounded by a CT capsule separating it from the thyroid
• 2 main cell types: chief cells and oxyphil cells
• Chief cells are dense clusters of small cells, they secrete PTH
• Oxyphil cells are bigger, more eosinophilic and fluffier and do not secrete PTH
• Oxyphil cells tend to cluster together, surrounded by many chief cells