L8 Thyroid and Parathyroid

Vocabulary flashcards based on the lecture notes about thyroid and parathyroid anatomy, hormone functions, and related conditions.

Thyroid gland structure

Butterfly-shaped with 2 lobes and an isthmus; located anterior to the trachea.

Thyroid follicles

Functional units filled with colloid (thyroglobulin, Tg).

Follicular cells

Produce Tg and thyroid hormones (T₃, T₄).

C cells (parafollicular cells)

Secrete calcitonin to regulate calcium.

Na⁺/I⁻ symporter

Transports iodide into follicular cells (TSH-stimulated).

Thyroperoxidase (TPO)

Enzyme that oxidizes I⁻ to iodine and couples tyrosines on Tg.

MIT (monoiodotyrosine)

1 iodine + tyrosine; precursor to T₃/T₄.

DIT (diiodotyrosine)

2 iodines + tyrosine; precursor to T₃/T₄.

T₃ (triiodothyronine)

Active hormone (MIT + DIT); 10x more potent than T₄.

T₄ (thyroxine)

Pro-hormone (DIT + DIT); converted to T₃ peripherally.

Pendrin

Exchanges I⁻ for Cl⁻ at follicular membrane.

Megalin

Binds iodinated Tg for endocytosis and hormone release.

Deiodinases

Convert T₄ to T₃ (Type I/II) or inactive rT₃ (Type III).

Thyroxine-binding globulin (TBG)

Binds 75% of circulating T₄.

Free hormone hypothesis

Only unbound T₃/T₄ is biologically active.

TRH (Thyrotropin-Releasing Hormone)

Hypothalamic hormone that stimulates TSH.

TSH (Thyroid-Stimulating Hormone)

Pituitary hormone that stimulates thyroid hormone synthesis.

Hashimoto’s thyroiditis

Autoimmune hypothyroidism (anti-TPO antibodies).

Graves’ disease

Autoimmune hyperthyroidism (TSH receptor antibodies).

Exophthalmos

Protruding eyes in Graves’ disease.

Parathyroid glands

4 small glands on thyroid’s posterior; secrete PTH.

PTH (parathyroid hormone)

Raises blood Ca²⁺ via bone, kidney, and gut actions.

Calcium-sensing receptor (CaSR)

Inhibits PTH when blood Ca²⁺ is high.

Calcitonin

Lowers blood Ca²⁺ (minor role in adults).

Hyperparathyroidism

High PTH → hypercalcemia, kidney stones, osteoporosis.

Hypoparathyroidism

Low PTH → hypocalcemia, tetany (CATS mnemonic).

Myxedema coma

Life-threatening severe hypothyroidism.

Levothyroxine

Synthetic T₄ for hypothyroidism treatment.

DiGeorge syndrome

Congenital absence of parathyroids → hypocalcemia.

Postpartum thyroiditis

Autoimmune thyroid inflammation after childbirth.