Thyroid Disorders for USMLE Step 3
Thyroid Disorders for the USMLE Step 3 Exam
Hyperthyroidism
- Pathophysiology
- Hyperthyroidism is characterized by excessive secretion of thyroid hormones (T3 and T4), leading to increased metabolic activity.
- Causes:
- Graves’ Disease: The most common cause, an autoimmune condition where thyroid-stimulating immunoglobulins (TSI) activate TSH receptors, causing diffuse thyroid enlargement and overproduction of hormones.
- Toxic Multinodular Goiter: Multiple hyperfunctioning thyroid nodules that secrete excess thyroid hormone.
- Toxic Adenoma: A single hyperfunctioning nodule autonomously secreting thyroid hormones.
- Thyroiditis: Inflammation of the thyroid (e.g., subacute thyroiditis) that releases stored thyroid hormones.
- Iatrogenic Hyperthyroidism: Due to excessive thyroid hormone supplementation or iodine exposure.
- Clinical Features
- General Symptoms: Weight loss despite increased appetite, heat intolerance, sweating, and fatigue.
- Cardiovascular: Tachycardia, atrial fibrillation, palpitations, and increased systolic blood pressure.
- Neurologic: Anxiety, tremor, irritability, insomnia, and hyperreflexia.
- Dermatologic: Warm, moist skin, fine hair, and onycholysis.
- Graves’ Disease-Specific:
- Exophthalmos (proptosis) and pretibial myxedema (thickening of the skin over the shins).
- Diagnosis
- Low TSH with elevated free T4/T3 confirms hyperthyroidism.
- Radioactive Iodine Uptake (RAIU) Scan:
- Graves’ Disease: Diffuse increased uptake.
- Toxic Multinodular Goiter/Toxic Adenoma: Focal increased uptake.
- Thyroiditis: Decreased uptake due to hormone leakage.
- Management
- Antithyroid Drugs:
- Methimazole: First-line for most patients.
- Propylthiouracil (PTU): Preferred during the first trimester of pregnancy and in thyroid storm.
- Beta-Blockers: Propranolol for symptomatic control (tachycardia, tremors).
- Radioactive Iodine Therapy: Destroys overactive thyroid tissue, commonly used for Graves’ disease.
- Surgery (Thyroidectomy): Reserved for patients with large goiters, malignancy suspicion, or treatment failure.
- Complications
- Thyroid Storm: A life-threatening condition with severe hyperthyroidism, fever, tachycardia, delirium, and cardiovascular collapse. Treatment includes PTU, beta-blockers, iodine, and corticosteroids.
Hypothyroidism
- Pathophysiology
- Hypothyroidism is due to insufficient production of thyroid hormones, resulting in a hypometabolic state.
- Causes:
- Hashimoto’s Thyroiditis: The most common cause, an autoimmune disorder where antibodies (anti-thyroid peroxidase [TPO] and anti-thyroglobulin) target the thyroid gland, causing destruction.
- Iatrogenic: Post-thyroidectomy, radioactive iodine ablation, or external radiation therapy.
- Medications: Lithium, amiodarone, and antithyroid drugs can cause hypothyroidism.
- Iodine Deficiency: Common globally, but rare in areas with sufficient dietary iodine.
- Central Hypothyroidism: Pituitary or hypothalamic dysfunction, leading to low TSH or thyrotropin-releasing hormone (TRH).
- Clinical Features
- General Symptoms: Fatigue, weight gain, cold intolerance, lethargy, and decreased appetite.
- Cardiovascular: Bradycardia, diastolic hypertension, and hypercholesterolemia.
- Neurologic: Depression, memory impairment, slowed reflexes, and mental fatigue.
- Dermatologic: Dry skin, brittle hair, and alopecia.
- Reproductive: Menstrual irregularities, infertility.
- Myxedema Coma: A severe form of hypothyroidism with hypothermia, altered mental status, and cardiovascular collapse.
- Diagnosis
- High TSH with low free T4 confirms primary hypothyroidism.
- Low TSH and low T4 suggest central hypothyroidism.
- Anti-TPO antibodies are elevated in Hashimoto’s thyroiditis.
- Management
- Levothyroxine (T4): The treatment of choice for hypothyroidism, titrated based on TSH levels, with monitoring every 6-8 weeks.
- Myxedema Coma: Requires emergency treatment with high-dose IV levothyroxine, supportive care, and corticosteroids.
- Complications
- Myxedema Coma: A life-threatening complication requiring urgent treatment with hormone replacement and intensive care.
Key Points
- Hyperthyroidism is commonly caused by Graves’ disease, toxic multinodular goiter, or toxic adenoma. It is characterized by weight loss, heat intolerance, and tachycardia. Diagnosis is confirmed by low TSH and high free T4/T3. Treatment includes antithyroid drugs, beta-blockers, radioactive iodine, or surgery.
- Thyroid Storm is a life-threatening exacerbation of hyperthyroidism requiring urgent intervention with PTU, beta-blockers, iodine, and corticosteroids.
- Hypothyroidism is often caused by Hashimoto’s thyroiditis. Symptoms include fatigue, weight gain, and cold intolerance. Diagnosis is confirmed by high TSH and low free T4. Levothyroxine is the treatment of choice.
- Myxedema Coma is a severe hypothyroid state requiring emergency treatment with IV levothyroxine and supportive care.