Page 549 - Concise Pathology for Exam Preparation ( PDFDrive )

20

Endocrinology

THYROID
• Weighs 15–20 g
• Has a rich intraglandular capillary network (from superior and inferior thyroid arteries)
and nerve supply from cervical sympathetic ganglia
• The gland is divided by thin fibrous septae into lobules composed of 20–40 evenly
dispersed follicles.
• Follicles are lined by cuboidal to low-columnar epithelium and contain thyroglobulin.
• Homoeostasis in the hypothalamus–pituitary–thyroid axis ensures maintenance of
normal thyroid functioning (Flowchart 20.1).

Hypothalamus
Release of thyrotropin-releasing hormone (TRH)
Thyrotrophs in anterior pituitary
Release of thyroid-stimulating hormone (TSH)

Active transport
Iodide Thyroid follicular cells

Iodination of tyrosine residues in thyroglobulin

Monoiodotyrosine (MIT) Diiodotyrosine (DIT)
MIT + DIT = T3
DIT + DIT = T4
T4 Peripheral deiodination T3
Free T4 = 0.03% of total T4
Free T3 = 0.3% of total T3
70% of T4 circulates in the peripheral blood bound to TBG (thyroid-binding globulin), 20% circulates
bound to TBPA (thyroid-binding proalbumin) and 10% to TBA (thyroid-binding albumin). Most T3 is
bound to TBG.
FLOWCHART 20.1. Mechanism of homoeostasis in the hypothalamus–pituitary–thyroid axis
and release of thyroid hormones.

534

mebooksfree.com