Thyroid Gland – T3, T4, Synthesis, Regulation, and Actions

What are the two main thyroid hormones and where are they produced?

T3 (Triiodothyronine)
and
T4 (Thyroxine)
Produced by the
thyroid follicular cells

What protein is central to thyroid hormone synthesis?

Thyroglobulin
– contains
tyrosine
residues that are iodinated

Describe the key steps in thyroid hormone synthesis.

1. Iodide uptake
   via Na⁺/I⁻ symporter
2. Iodination
   of tyrosine → MIT & DIT
3. Coupling:MIT + DIT → T3
   DIT + DIT → T4
4. Endocytosis and proteolysis of thyroglobulin → Release of T3/T4

What enzyme catalyzes iodination and coupling in the thyroid?

Thyroid peroxidase (TPO)

What is the ratio of T4 to T3 release from the thyroid?

~ 5:1, but T3 is the more potent form

How is T4 activated to T3 in peripheral tissues?

Via 5'-deiodination by selenodeiodinases

What regulates T3/T4 secretion?

↓ T3/T4 → ↑ TRH (hypothalamus) → ↑ TSH (pituitary) → ↑ T3/T4
↑ T3/T4 → Negative feedback on TRH and TSH

What are the physiological effects of T3 and T4?

↑ Basal metabolic rate and body temperature
↑ Heart rate and cardiac output
↑ Growth and CNS development
↑ LDL receptor expression and bile acid synthesis
↑ Gluconeogenesis, lipolysis

What is the effect of thyroid hormones on glucose metabolism?

↑ Gluconeogenesis
↑ Amino acid supply
↑ Glycerol supply (via lipolysis)

What are the main plasma-binding

TBG (Thyroxine-binding globulin)
Albumin
Transthyretin (for T4)