Thyroid Hormones: Analogues and Their Inhibitors

• Thyroid Hormones: Analogues and Their Inhibitors include levothyroxine, liothyronine, and antithyroid drugs.
• Thyroid Hormones: Analogues and Their Inhibitors regulate metabolism and treat thyroid disorders.
• Thyroid hormones play a crucial role in metabolism, growth, and development.
• Pharmacological manipulation of these hormones is essential in treating various thyroid disorders.

Thyroid Hormones: T3 (Triiodothyronine) and T4 (Thyroxine)

Synthesis:

1. Iodide uptake by thyroid follicular cells.
2. Iodination of tyrosine residues on thyroglobulin (organification).
3. Coupling of iodotyrosines (MIT & DIT) to form T3 and T4.
4. Proteolysis of thyroglobulin to release T3 and T4 into circulation.

Regulation:

• TSH from the pituitary stimulates thyroid hormone synthesis.
• Negative feedback by circulating T3/T4 regulates secretion.

Thyroid Hormone Analogues

1. Levothyroxine (Synthetic T4):

   • Uses: Primary treatment for hypothyroidism.
   • Mechanism: Replaces deficient T4, converted to active T3 in peripheral tissues.
   • Adverse Effects: Symptoms of hyperthyroidism if overdosed (e.g., tachycardia, weight loss).

2. Liothyronine (Synthetic T3):

   • Uses: Rapid thyroid hormone action (e.g., myxedema coma).
   • Adverse Effects: Higher risk of cardiac side effects.

3. Desiccated Thyroid Extracts:

   • Uses: Contains both T4 and T3, less commonly used.

Thyroid Hormone Inhibitors

1. Thionamides:

   • Methimazole and Propylthiouracil (PTU):
   • Uses: First-line treatments for hyperthyroidism (e.g., Graves’ disease).
   • Mechanism: Inhibit thyroid peroxidase, blocking iodine incorporation and coupling. PTU also inhibits peripheral T4 to T3 conversion.
   • Adverse Effects: Rash, agranulocytosis, hepatotoxicity (rare).

2. Iodine Solutions (Potassium Iodide):

   • Uses: Acute hyperthyroidism management (e.g., thyroid storm) and pre-surgical preparation.
   • Mechanism: Temporarily inhibits hormone release and synthesis (Wolff–Chaikoff effect).
   • Adverse Effects: Hypersensitivity, metallic taste, salivary gland swelling.

3. Radioactive Iodine (I-131):

   • Uses: Definitive hyperthyroidism treatment by destroying thyroid tissue.
   • Adverse Effects: Hypothyroidism post-treatment, radiation thyroiditis.

4. Beta-Blockers (e.g., Propranolol):

   • Uses: Symptomatic control (e.g., tachycardia, tremor) and blocks peripheral T4 to T3 conversion.
   • Adverse Effects: Bradycardia, fatigue.

Clinical Applications

• Hypothyroidism: Treated with levothyroxine or liothyronine to restore normal function.
• Hyperthyroidism: Managed with thionamides, iodine solutions, radioactive iodine, or beta-blockers.
• Thyroid Cancer: Treated with thyroid hormone analogues to suppress TSH or radioactive iodine for ablation.

Side Effects Overview

• Thionamides: Agranulocytosis, hepatotoxicity, teratogenicity (methimazole).
• Levothyroxine: Hyperthyroidism symptoms if overdosed (e.g., palpitations, anxiety).
• Radioactive Iodine: Hypothyroidism post-treatment, radiation thyroiditis.

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