The thyroid gland produces hormones that regulate metabolism, growth, development, and energy balance. Disorders of thyroid function include hypothyroidism (low hormone levels) and hyperthyroidism (excess hormone levels). Thyroid and antithyroid drugs help restore hormonal balance.
Thyroid Hormones
- Thyroxine (T4)
- Triiodothyronine (T3)
T4 is converted to T3 inside tissues. T3 is the more active hormone. Both hormones contain iodine and are stored in the thyroid gland as part of the protein thyroglobulin.
Functions of Thyroid Hormones
- Increase basal metabolic rate
- Regulate protein, carbohydrate, and lipid metabolism
- Essential for growth and brain development
- Increase heart rate and oxygen consumption
Synthesis of Thyroid Hormones (Simplified)
- Iodine uptake by thyroid gland
- Oxidation of iodide to iodine
- Iodination of tyrosine residues on thyroglobulin
- Coupling to form T3 and T4
- Release into bloodstream
Hypothyroidism
A condition of low thyroid hormone levels.
Causes
- Hashimoto’s thyroiditis (autoimmune destruction)
- Iodine deficiency
- Congenital hypothyroidism
- Thyroidectomy or radiation
Symptoms
- Fatigue, weight gain
- Cold intolerance
- Slow heart rate
- Dry skin
- Myxedema (severe form)
Treatment – Thyroid Replacement Therapy
1. Levothyroxine (T4)
Most commonly used drug for hypothyroidism.
Advantages: Long half-life, stable, predictable action.
2. Liothyronine (T3)
Faster onset but short duration; used when rapid correction is needed.
3. Desiccated Thyroid Extract
Older preparation; rarely used due to variable potency.
Hyperthyroidism
A condition of excess thyroid hormone production. Common causes include Graves’ disease, toxic nodular goiter, and excessive iodine intake.
Symptoms
- Weight loss despite normal appetite
- Heat intolerance
- Tremors and anxiety
- Rapid heartbeat (tachycardia)
- Bulging eyes (in Graves’ disease)
Antithyroid Agents
Drugs used to reduce thyroid hormone production in hyperthyroidism.
1. Thiourea Derivatives (Thionamides)
They inhibit thyroid hormone synthesis by blocking iodination and coupling reactions.
- Propylthiouracil (PTU)
- Methimazole
- Carbimazole (converted to methimazole in the body)
Mechanism of PTU
- Blocks iodination and coupling in thyroid gland
- Also inhibits conversion of T4 to T3 in peripheral tissues
Adverse Effects
- Rashes, itching
- Agranulocytosis (rare but serious)
- Liver toxicity (especially with PTU)
2. Iodides
High doses of iodine temporarily inhibit thyroid hormone release (Wolff–Chaikoff effect).
- Potassium iodide
- Lugol’s iodine
Uses
- Before thyroid surgery (reduces gland size)
- Thyroid storm
3. Radioactive Iodine (I-131)
Destroys thyroid tissue by emitting β-radiation.
Uses
- Treatment of Graves’ disease
- Toxic nodular goiter
Contraindication
Not used in pregnancy or children.
4. Beta Blockers
These do not lower hormone levels but relieve symptoms of hyperthyroidism.
- Propranolol
Uses: Reduces tremors, palpitations, and anxiety.
Diagnostic Agents
- Radioactive iodine uptake test – measures gland activity
- TSH test – differentiates primary from secondary thyroid disorders
Detailed Notes:
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