THYROID AGENTS

1. Introduction

Thyroid agents are drugs that either replace thyroid hormones (in hypothyroidism) or inhibit thyroid hormone synthesis (in hyperthyroidism).
They help maintain normal metabolism, growth, and development.

2. Classification

A. Thyroid Hormone Preparations

Used in hypothyroidism (e.g., Hashimoto’s disease, post-thyroidectomy, or cretinism):

• Levothyroxine (T₄) – synthetic T₄; drug of choice.

• Liothyronine (T₃) – faster onset, shorter half-life.

• Liotrix – combination of T₄ and T₃.

• Desiccated thyroid – dried animal thyroid gland (rarely used).

Mechanism of Action:
→ Converted to T₃ → binds nuclear receptors → increases transcription of metabolic enzymes → ↑ basal metabolic rate.

Pharmacokinetics:

• Absorption: oral (best on empty stomach).

• Onset: slow (1–3 weeks).

• Half-life: long (T₄ ≈ 7 days).

Indications:

• Hypothyroidism

• Myxedema coma (IV T₃)

• Goiter (non-toxic)

• Post-thyroidectomy therapy

Adverse Effects (overdose → hyperthyroid state):

• Tachycardia, palpitations

• Weight loss, tremor, heat intolerance

• Insomnia, anxiety

• Atrial fibrillation (elderly)

Contraindications:

• Untreated adrenal insufficiency

• Thyrotoxicosis

• Caution in cardiac disease

B. Antithyroid Drugs (Thioamides)

Used in hyperthyroidism (e.g., Graves’ disease):

Adverse Effects:

• Agranulocytosis

• Rash, urticaria

• Hepatotoxicity (PTU)

• Arthralgia

Other agents used in thyrotoxicosis:

• Iodide (Lugol’s iodine, potassium iodide): inhibits hormone release.

• Radioactive iodine (¹³¹I): destroys thyroid tissue.

• Beta-blockers (Propranolol): control symptoms, inhibit T₄→T₃ conversion.

• Glucocorticoids: reduce T₄→T₃ conversion and manage storm.

🦴 PARATHYROID MEDICATIONS

1. Introduction

The parathyroid glands regulate calcium and phosphate via PTH (parathyroid hormone).
Disorders include hypoparathyroidism (↓PTH → hypocalcemia) and hyperparathyroidism (↑PTH → hypercalcemia).

2. Classification

A. Parathyroid Hormone & Analogs

Used in hypocalcemia / osteoporosis:

• Teriparatide (PTH 1-34): recombinant PTH fragment – stimulates bone formation (intermittent use).

• Abaloparatide: PTHrP analog, same action.

Adverse Effects:

• Hypercalcemia, hyperuricemia, osteosarcoma risk (long-term).

B. Vitamin D and Analogs

Used in rickets, osteomalacia, renal osteodystrophy, hypocalcemia:

• Cholecalciferol (D₃)

• Ergocalciferol (D₂)

• Calcitriol (1,25(OH)₂D₃): active form, used in renal failure.

• Alfacalcidol: precursor activated in liver.

Mechanism: ↑ calcium & phosphate absorption from gut, ↓ renal excretion.

C. Calcium Supplements

Used in hypocalcemia:

• Calcium carbonate

• Calcium citrate

• Calcium gluconate (IV)

D. Calcimimetics

Used in secondary hyperparathyroidism (CKD) and parathyroid carcinoma:

• Cinacalcet

• Etelcalcetide (IV form)

Mechanism: Increases Ca-sensing receptor sensitivity → ↓ PTH secretion.

Adverse Effects: Hypocalcemia, nausea, vomiting.

E. Calcitonin

Produced by thyroid C-cells; opposes PTH.

Drugs: Salmon calcitonin, human calcitonin.

Uses:

• Hypercalcemia

• Paget’s disease

• Osteoporosis (less common now)

3. Summary Table

4. Clinical Correlations

• Myxedema coma: IV levothyroxine + hydrocortisone.

• Thyroid storm: PTU + propranolol + iodine + steroids.

• Post-thyroidectomy tetany: IV calcium gluconate.

• Chronic renal disease: use calcitriol + phosphate binders + cinacalcet.