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EndocrinE ` endocrine—Physiology EndocrinE ` endocrine—Physiology SEcTion iii 331

Thyroid hormones Thyroid produces triiodothyronine (T 3 ) and thyroxine (T 4 ), iodine-containing hormones that
control the body’s metabolic rate.

soUrce Follicles of thyroid. 5′-deiodinase converts T 4 (the major thyroid product) to T 3 in peripheral tissue
(5, 4, 3). Peripheral conversion is inhibited by glucocorticoids, β-blockers, and propylthiouracil
(PTU). Reverse T 3 (rT 3 ) is a metabolically inactive byproduct of the peripheral conversion of T 4
and its production is increased by growth hormone and glucocorticoids. Functions of thyroid
peroxidase include oxidation, organification of iodine, and coupling of monoiodotyrosine (MIT)
and diiodotyrosine (DIT). Inhibited by PTU and methimazole. DIT + DIT = T 4 . DIT + MIT =
T 3 . Wolff-Chaikoff effect—excess iodine temporarily turns off thyroid peroxidase Ž  T 3 /T 4
production (protective autoregulatory effect).
FUnction Only free hormone is active. T 3 binds nuclear receptor with greater affinity than T 4 . T 3 functions
—7 B’s:
ƒ Brain maturation
ƒ Bone growth (synergism with GH)
ƒ β-adrenergic effects.  β 1 receptors in heart Ž  CO, HR, SV, contractility; β-blockers alleviate
adrenergic symptoms in thyrotoxicosis
+
+
ƒ Basal metabolic rate  (via Na /K -ATPase activity Ž  O 2 consumption, RR, body temperature)
ƒ Blood sugar ( glycogenolysis, gluconeogenesis)
ƒ Break down lipids ( lipolysis)
ƒ Stimulates surfactant synthesis in Babies
regUlAtion TRH ⊕ TSH release Ž ⊕ follicular cells. Thyroid-stimulating immunoglobulin (TSI) may ⊕
follicular cells in Graves disease.
Negative feedback primarily by free T 3 /T 4 :
ƒ Anterior pituitary Ž  sensitivity to TRH
ƒ Hypothalamus Ž  TRH secretion
Thyroxine-binding globulin (TBG) binds most T 3 /T 4 in blood. Bound T 3 /T 4 = inactive.
ƒ  TBG in pregnancy, OCP use (estrogen Ž  TBG) Ž  total T 3 /T 4
ƒ  TBG in steroid use, nephrotic syndrome

Hypothalamus
Peripheral tissue Blood Thyroid follicular epithelial cell Follicular lumen
TRH
TG TG
Tyrosine
Thyroglobulin
+
Anterior pituitary I – I – Oxidation I 2
Somatostatin Na + Organification
TSH Downstream thyroid Thyroid
function Deiodinase peroxidase MIT
PTU, TG MIT
methimazole DIT
MIT, DIT DIT
Thyroid follicular cells
Thyroid Coupling
peroxidase reaction
TSI T
T , T 3 MIT MIT
DIT
3 4 T T T > T TG DIT TG DIT
3 4 4 3 T 3 T 3
5'-deiodinase (to circulation) Proteases Endocytosis
T 4 T 4
PTU

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