12. THYROID DISEASES

The thyroid is a small, butterfly-shaped gland located in front of the windpipe. It produces the hormones T3 (triiodothyronine) and T4 (thyroxine), which control metabolism, heat production, and how the body uses energy. The pituitary gland controls thyroid function by releasing thyroid-stimulating hormone (TSH).

Thyroid Hormone Physiology (Simple Explanation)

Iodine enters thyroid cells and combines with tyrosine on a large protein called thyroglobulin. Two iodinated tyrosines join to form T4, and one MIT + one DIT form T3. T3 is more active than T4.

Most T4 and T3 in blood are bound to proteins such as TBG and albumin, but only free hormones are active. The thyroid makes all of the T4, but most T3 is formed by conversion of T4 in tissues. TSH from the pituitary stimulates hormone production. High T3/T4 suppress TSH (negative feedback).

THYROTOXICOSIS (HYPERTHYROIDISM)

Hyperthyroidism is a condition where the thyroid makes too much hormone, increasing metabolism and overstimulating body systems. It is more common in women. The most common cause is Graves’ disease.

Etiology

  • Graves’ disease: autoimmune stimulation of the thyroid causing diffuse enlargement.
  • Toxic nodules: single or multiple overactive nodules (toxic adenoma or multinodular goiter).
  • Thyroiditis: painful or painless release of stored hormones.
  • Excess iodine: from drugs like amiodarone or iodine-rich supplements.
  • TSH-secreting pituitary tumors (rare).
  • Excess thyroid hormone intake (thyrotoxicosis factitia).

Pathophysiology

In Graves’ disease, antibodies stimulate the TSH receptor causing continuous hormone production. Toxic nodules produce hormone independently of pituitary control. Thyroiditis releases preformed hormones.

Clinical Features

  • Nervousness, anxiety, irritability
  • Palpitations, fast heartbeat
  • Heat intolerance, sweating
  • Weight loss despite increased appetite
  • Frequent bowel movements
  • Muscle weakness, fatigue
  • Moist skin, fine hair, tremors
  • Eye changes in Graves’ disease (exophthalmos)

Diagnosis (Both Hyper & Hypothyroid)

1. Blood Tests

  • TSH: low in hyperthyroidism, high in hypothyroidism.
  • T4 and Free T4: high in hyperthyroidism, low in hypothyroidism.
  • T3 and Free T3: helpful in diagnosing severity.
  • Thyroid antibodies: TPO, Tg, TSI for autoimmune diseases.
  • Calcitonin and thyroglobulin: used in cancer and thyroiditis evaluation.

2. Imaging Tests

  • Thyroid scan: evaluates size, shape, and nodules.
  • Ultrasound: detects enlargement and nodules without radiation.

Treatment of Hyperthyroidism

Goals

Reduce excess thyroid hormone, relieve symptoms, and prevent complications.

Non-Pharmacological Therapy

  • Thyroidectomy for very large goiters or when drugs fail.
  • Pre-surgery: thionamides + iodides to shrink and stabilize the gland.
  • Propranolol for symptom control before and after surgery.

Pharmacological Therapy

1. Thionamides (PTU & Methimazole)

  • Block thyroid hormone synthesis.
  • PTU also blocks conversion of T4 to T3.
  • Used for 12–24 months to induce remission.
  • Major ADRs: agranulocytosis, hepatotoxicity.

2. Iodides

  • Rapidly block hormone release and shrink the gland.
  • Used before surgery or after radioactive iodine therapy.
  • Given as SSKI or Lugol’s iodine.

3. Beta-Blockers

  • Control symptoms: palpitations, tremor, anxiety.
  • Propranolol also slightly reduces T4→T3 conversion.

4. Radioactive Iodine (RAI)

  • Destroys overactive thyroid cells.
  • Contraindicated in pregnancy.
  • Often causes hypothyroidism requiring lifelong levothyroxine.

Thyroid Storm

A life-threatening emergency caused by extremely high thyroid hormone levels. Symptoms: high fever, severe tachycardia, confusion, dehydration, vomiting, and diarrhea.

Treatment:

  • Thionamides followed by iodides
  • Beta-blockers (esmolol preferred)
  • IV corticosteroids
  • Supportive care (fluids, antipyretics, antibiotics if needed)

HYPOTHYROIDISM

Hypothyroidism occurs when the thyroid produces too little hormone, slowing metabolism and body functions.

Etiology

  • Hashimoto’s thyroiditis (autoimmune destruction)
  • Post-surgery or post-radioactive iodine
  • Iodine deficiency
  • Thyroiditis (postpartum or inflammatory)
  • Pituitary failure (secondary hypothyroidism)

Clinical Features

  • Fatigue, weight gain
  • Cold intolerance
  • Dry skin, coarse hair
  • Constipation
  • Slow heart rate
  • Depression, memory issues
  • Myxedema coma (severe emergency)

Treatment of Hypothyroidism

Levothyroxine (T4) — Drug of Choice

  • Stable, predictable, and converted to active T3 in the body.
  • Young adults: start with 50 mcg/day → increase to 100 mcg in 1 month.
  • Elderly or heart disease: start 25 mcg/day → increase slowly.
  • Maintenance dose: around 125 mcg/day (varies by person).
  • Avoid switching brands due to potency differences.

Drug Interactions

  • Reduced absorption with calcium, iron, antacids, sucralfate, fiber.
  • Increased clearance with rifampin, carbamazepine, phenytoin.
  • Amiodarone interferes with T4→T3 conversion.

Treatment of Myxedema Coma

  • IV levothyroxine 300–500 mcg + IV hydrocortisone
  • Supportive therapy (ventilation, temperature, BP stabilization)
  • Oral therapy once stable

Detailed Notes:

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