Thyroid Disorders for USMLE Step 2

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Thyroid Disorders for the USMLE Step 2 Exam

Hyperthyroidism

Hyperthyroidism occurs due to excessive production of thyroid hormones (T3 and T4), causing increased metabolic activity.
Common Causes:
Graves’ Disease: An autoimmune disorder where thyroid-stimulating immunoglobulins (TSI) activate TSH receptors, leading to diffuse thyroid enlargement and overproduction of thyroid hormones.
Toxic Multinodular Goiter: Multiple hyperfunctioning thyroid nodules producing excess thyroid hormones.
Toxic Adenoma: A single hyperfunctioning thyroid nodule.
Thyroiditis: Transient hyperthyroidism due to thyroid inflammation and hormone release (e.g., subacute thyroiditis).
Iatrogenic Hyperthyroidism: Over-replacement with thyroid hormone or iodine excess.

General Symptoms: Weight loss despite increased appetite, heat intolerance, sweating, palpitations, and fatigue.
Cardiovascular: Tachycardia, atrial fibrillation, and increased systolic blood pressure.
Neurologic: Tremors, hyperreflexia, anxiety, and irritability.
Graves' Disease-Specific Features:
Exophthalmos (proptosis) and pretibial myxedema (thickening of the skin over the shins).
Gastrointestinal: Diarrhea and increased bowel movements.

Antithyroid Medications:
Methimazole (first-line) or propylthiouracil (PTU), especially during the first trimester of pregnancy.
Beta-blockers: Propranolol to control adrenergic symptoms.
Radioactive Iodine Ablation: Destroys overactive thyroid tissue, commonly used in Graves’ disease.
Thyroidectomy: Indicated for large goiters, thyroid cancer, or when other treatments fail.

Thyroid Storm: Life-threatening hyperthyroidism with fever, tachycardia, and altered mental status, requiring urgent treatment with PTU, beta-blockers, iodine, and corticosteroids.

Hypothyroidism

Hypothyroidism is due to decreased production of thyroid hormones, leading to a hypometabolic state.
Common Causes:
Hashimoto’s Thyroiditis: Autoimmune destruction of the thyroid gland, causing progressive thyroid failure.
Iatrogenic: After thyroidectomy or radioactive iodine ablation.
Medications: Lithium, amiodarone.
Iodine Deficiency: In regions with low dietary iodine.
Central Hypothyroidism: Due to pituitary or hypothalamic disorders, leading to low TSH or TRH.

General Symptoms: Fatigue, weight gain, cold intolerance, and lethargy.
Dermatologic: Dry skin, brittle hair, and hair loss.
Cardiovascular: Bradycardia, diastolic hypertension, and hyperlipidemia.
Neurologic: Depression, cognitive slowing, and delayed reflexes.
Gastrointestinal: Constipation and decreased appetite.
Myxedema Coma: Severe hypothyroidism characterized by hypothermia, altered mental status, and cardiovascular collapse.

Thyroid Function Tests:
High TSH and low free T4 confirm primary hypothyroidism.
Low TSH with low free T4 suggests central hypothyroidism.
Autoantibodies: Elevated anti-thyroid peroxidase (TPO) antibodies in Hashimoto’s thyroiditis.

Levothyroxine: Synthetic T4 is the treatment of choice. Doses are titrated based on TSH levels, with monitoring every 6-8 weeks.
Myxedema Coma: An emergency requiring IV levothyroxine, supportive care, and corticosteroids to manage possible adrenal insufficiency.

Myxedema Coma: A life-threatening emergency caused by severe hypothyroidism, treated with high-dose IV hormone replacement and supportive measures.

Key Points

Hyperthyroidism is commonly caused by Graves' disease, toxic multinodular goiter, or toxic adenoma. Symptoms include weight loss, heat intolerance, and tachycardia. It is diagnosed by low TSH and high T3/T4 levels.
Management includes antithyroid drugs, beta-blockers, radioactive iodine, and surgery. Thyroid storm is a severe complication requiring emergency treatment.
Hypothyroidism is often caused by Hashimoto’s thyroiditis. Symptoms include fatigue, cold intolerance, and bradycardia. Diagnosis is made by elevated TSH and low free T4 levels.
Levothyroxine is the treatment for hypothyroidism, with monitoring of TSH levels for dose adjustments.