Pharmacology of Thyroid Hormone

Pharmacology of Thyroid Hormone

• Thyroid hormone synthesis: Thyroid cells take up iodine and couple it with tyrosine residues in thyroglobulin. This creates thyroxine (T4) and triiodothyronine (T3).

• Hyposecretion (Hypothyroidism): Thyroid replacement therapy includes levothyroxine sodium (T4), typically more effective than liothyronine sodium (T3) in suppressing TSH.

• Hypersecretion (Hyperthyroidism): Treatments include antithyroid drugs, irradiation, or surgical removal of the overactive tissue.

Thyroid Hormone Preparations

• Levothyroxine and Liothyronine: Synthetic thyroid hormones used for replacement therapy. A mixture containing T4 and T3 in a 4:1 ratio (liotirix) is also available. Levothyroxine is the typical drug of choice for hypothyroidism.

• Pharmacokinetics: T4 and T3 have good absorption and are adequately absorbed orally. T4 has a longer half-life (one week) compared to T3 (two days).

Antithyroid Agents

• Thioamide drugs: Carbimazole, methimazole, and propylthiouracil (PTU). PTU is often preferred during pregnancy and lactation.

• Indicate use : used to treat Graves' disease & suppress symptoms before surgery or radioactive iodine treatment.

• Pharmacokinetics Given orally; carbimazole is metabolized to methimazole.

Other Anti-thyroid Agents

• Beta-adrenergic antagonists: Like propranolol, reduce the cardiovascular effects of hyperthyroidism (e.g., anxiety).

• Iodide salts: Immediate effects on thyroid hormone release, used in thyroid storm (acute thyrotoxicosis) and short-term treatment.

Radioactive Iodine

• Mechanism: Absorbed rapidly by the thyroid and emits beta particles to destroy thyroid tissue.

• Uses: Definitive treatment for relapses of Graves' disease, toxic multinodular goiter and toxic solitary nodule.

• Contraindications: Pregnancy and lactation.

Bone Disorders

• Osteoporosis: Reduced bone mass, leading to easy fractures. Common in postmenopausal women.

• Osteomalacia: Abnormal bone mineralization, often caused by vitamin D deficiency. Similar in children, results in rickets.

Drugs Affecting Calcium and Bone

• Cinacalcet: Activates calcium sensing receptors, lowering parathyroid hormone levels, reducing calcium levels.

• Calcitonin: Decreases calcium excretion by the kidneys and calcium absorption in the intestines; used for managing hypercalcemia and osteoporosis.

• Vitamin D: A crucial nutrient for calcium regulation; its activation converts vitamin D to its active form (calcitriol).

• Diet is an important preventative measure in maintaining a balance of bone health.

Drug Treatment of Bone Loss

• Bisphosphonates: Inhibit bone resorption, commonly used in osteoporosis prevention and treatment.

Strontium Ranelate

• Usage: Indicated for prevention and treatment of osteoporosis in postmenopausal women.

• Mechanism: Decreases osteoclastic activity and increases bone formation.

Sodium Fluoride

• Mechanism: Replaces hydroxyl groups in bone, to form fluorapatite, which is more resistant to erosion than hydroxyapatite.

• Usage: Prevents dental caries and tooth decay.

Denosumab

• Mechanism: Inhibits osteoclast activity, reducing bone resorption.

• Usage: Treats osteoporosis

Teriparatide

• Mechanism: Newer drug, a type of parathyroid hormone.

• Usage: Treats osteoporosis.

Calcium and Vitamin D

• Adequate intake: Critical for bone health in children and adults to prevent osteoporosis, primarily by consuming dairy products and taking supplements when necessary,

• Potential issues: Aging, malabsorption issues or certain medications can disrupt vitamin D metabolism, leading to bone-related issues.

Iron-Deficiency and Anemia

• Iron-deficiency Anemia (IDA): Results from insufficient iron uptake.

• Anemia of Inflammation (AI/ACD): Associated with chronic inflammation or disease.