🦋 Thyroid Hormone - Synthesis & Secretion

📍 Pehle Samjho - Thyroid Gland Kya Hai?

⚙️ SYNTHESIS - Step by Step

Step 1️⃣ - Iodide Uptake (I⁻ ka andar aana)

• Blood se iodide (I⁻) thyrocyte ke andar aata hai ek special transporter ke through
• Ye transporter hai NIS - Sodium-Iodide Symporter
• Ye 2 Na⁺ ke saath ek baar mein 1 I⁻ transport karta hai (secondary active transport)
• Iodide plasma se 20-40x zyada concentrate hota hai thyroid mein
• TSH is process ko stimulate karta hai
• NIS salivary glands, gastric mucosa, placenta mein bhi hota hai - lekin wahan TSH se affect nahi hota

Step 2️⃣ - Thyroglobulin (Tg) ki Synthesis

• Thyrocytes thyroglobulin banate hain - ek bada glycoprotein (2 subunits se bana)
• Isme 123 tyrosine residues hote hain
• Lekin sirf 4-8 tyrosines hi normally thyroid hormones mein incorporate hoti hain
• Tg ko exocytosis ke through colloid mein secrete kiya jata hai

Step 3️⃣ - Organification (Iodination of Tyrosine)

• Thyrocyte ki apical membrane par Pendrin channel se I⁻ colloid mein release hota hai
• Wahan Thyroid Peroxidase (TPO) enzyme iodide ko oxidize karke reactive iodine (I₂) banata hai - H₂O₂ ki help se
• Ye reactive iodine Tg ke tyrosine residues se react karta hai
• Result:
  • MIT - Monoiodotyrosine (ek iodine laga)
  • DIT - Diiodotyrosine (do iodine lage)

💊 Clinical Point: PTU (Propylthiouracil) aur Methimazole - dono TPO ko block karte hain → hormone synthesis ruk jati hai

Step 4️⃣ - Coupling (MIT + DIT ka milna)

Normal thyroid distribution: MIT 3%, DIT 33%, T4 35%, T3 7%

Step 5️⃣ - Storage

• T3 aur T4 thyroglobulin se bound rehte hain colloid mein
• Colloid ek reservoir hai - agar aap 2 mahine tak bilkul bhi iodine na lo, tab bhi circulating thyroid hormone level normal rahega!

🚀 SECRETION - Hormones Ka Release

Colloid Endocytosis

• Jab hormones ki zaroorat hoti hai → TSH signal milta hai
• Thyrocytes colloid ko endocytosis se andar le aate hain
• Lysosomes Tg ko proteolysis se tod dete hain
• Free T4 aur T3 cytosol mein release hote hain → phir capillaries mein enter karte hain

Kitna secrete hota hai (per day)?

MIT aur DIT secrete nahi hote - inhe iodotyrosine deiodinase deiodinate kar deta hai aur iodine recycle ho jata hai

🔄 REGULATION - HPT Axis

1. Hypothalamus → TRH (Thyrotropin-Releasing Hormone) release karta hai
   • TRH ek tripeptide hai, paraventricular nuclei se
   • Anterior pituitary ke thyrotrophs par act karta hai
2. Anterior Pituitary → TSH (Thyroid-Stimulating Hormone) release karta hai
   • TSH ek glycoprotein hai
   • Thyroid ke har step ko stimulate karta hai: I⁻ uptake, oxidation, organification, coupling, endocytosis, proteolysis - sab kuch!
   • TSH ka second messenger: cAMP (Gs protein coupled)
   • TSH se thyroid mein hypertrophy + hyperplasia bhi hoti hai
3. Negative Feedback:
   • High T3/T4 → Pituitary par TRH receptor down-regulate ho jata hai
   • Feedback free T3 se hoti hai (kyunki pituitary mein deiodinase hota hai jo T4→T3 convert karta hai)
   • Result: steady TSH → steady thyroid hormone secretion

⚠️ Stimulatory vs Inhibitory Factors

🩺 T4 vs T3 - Important Points

• Thyroid mostly T4 secrete karta hai (80 µg/day) - jo less active hai
• Target tissues mein 5'-deiodinase T4 ko T3 mein convert karta hai (outer ring se 1 iodine hata ke) - active form
• Reverse T3 (rT3) - inner ring se iodine hatne se banta hai - inactive
• Starvation mein: 5'-deiodinase inhibit hoti hai (brain mein nahi) → BMR kam hota hai → energy bachti hai

🧬 Transport in Blood

• Circulating T3/T4 mostly Thyroxine-Binding Globulin (TBG) se bound rahte hain
• Sirf free hormones biologically active hain
• Sirf free hormones feedback mein participate karte hain
• TBG ka kaam: hormones ko degradation se protect karna, uniform tissue distribution

🔑 Yaad Rakhne Wali Key Points

1. NIS = iodide uptake (TSH stimulated)
2. TPO = organification + coupling (PTU/Methimazole block karte hain)
3. MIT + DIT → T3; DIT + DIT → T4
4. Colloid = hormone reservoir (2 mahine ka buffer)
5. T4 prohormone hai, T3 active form hai
6. Free T3 negative feedback deta hai pituitary par
7. Wolff-Chaikoff effect = excess iodide synthesis temporarily block karta hai
8. Graves disease = TSH receptor antibodies → hyperthyroidism