Thyroid Disorders for USMLE Step 1

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Thyroid Disorders for the USMLE Step 1 Exam

Hyperthyroidism

Hyperthyroidism is characterized by excessive production of thyroid hormones (T3 and T4), leading to a hypermetabolic state.
Causes:
Graves’ Disease: An autoimmune disorder where thyroid-stimulating immunoglobulins (TSI) activate TSH receptors, resulting in diffuse thyroid enlargement and hormone overproduction.
Toxic Multinodular Goiter (Plummer’s Disease): Hyperfunctioning nodules produce excess thyroid hormones independent of TSH.
Toxic Adenoma: A single autonomously functioning nodule producing excessive hormones.
Thyroiditis: Inflammatory conditions (e.g., subacute or silent thyroiditis) lead to thyroid hormone release.
Iatrogenic: Excessive exogenous thyroid hormone replacement or iodine exposure.

General Symptoms: Weight loss, heat intolerance, increased appetite, sweating, and fatigue.
Cardiovascular: Palpitations, tachycardia, atrial fibrillation, and hypertension.
Neurologic: Tremors, anxiety, restlessness, and hyperreflexia.
Gastrointestinal: Increased bowel movements, diarrhea.
Graves' Disease-Specific Features:
Exophthalmos: Proptosis and lid lag.
Pretibial Myxedema: Thickened skin over the shins.

Antithyroid Medications:
Methimazole is preferred but Propylthiouracil (PTU) is used in pregnancy.
Beta-blockers: Propranolol controls adrenergic symptoms.
Radioactive Iodine Ablation: Destroys thyroid tissue in hyperthyroidism.
Thyroidectomy: Reserved for refractory cases or large goiters.

Thyroid Storm: A life-threatening emergency with fever, tachycardia, delirium, and severe hyperthyroidism symptoms. Treated with PTU, beta-blockers, iodine, and steroids.

Hypothyroidism

Hypothyroidism is characterized by inadequate thyroid hormone production.
Causes:
Hashimoto’s Thyroiditis: Autoimmune destruction of the thyroid gland, causing gradual thyroid failure.
Iatrogenic Hypothyroidism: Post-thyroidectomy or radioactive iodine ablation.
Medications: Lithium and amiodarone.
Iodine Deficiency: A global cause of hypothyroidism.
Central Hypothyroidism: Pituitary or hypothalamic dysfunction causing decreased TSH or TRH.

General Symptoms: Fatigue, weight gain, cold intolerance, lethargy, and decreased appetite.
Dermatologic: Dry skin, hair loss, and brittle nails.
Cardiovascular: Bradycardia, diastolic hypertension, and hypercholesterolemia.
Neurologic: Depression, memory impairment, and delayed reflexes.
Reproductive: Menstrual irregularities, infertility.
Myxedema Coma: A severe form of hypothyroidism presenting with hypothermia, altered mental status, and cardiovascular collapse.

High TSH and low free T4 confirm primary hypothyroidism.
Anti-TPO and anti-thyroglobulin antibodies are elevated in Hashimoto’s thyroiditis.
Low TSH and low free T4 suggest central hypothyroidism.

Levothyroxine: Synthetic T4 is the treatment of choice. Dosing is based on TSH levels.
Myxedema Coma: Treated with IV levothyroxine and corticosteroids.

Myxedema Coma: A life-threatening emergency requiring rapid hormone replacement and supportive care.

Key Points

Hyperthyroidism is most often caused by Graves’ disease, toxic multinodular goiter, or toxic adenoma. It is diagnosed by low TSH and elevated T3/T4. Management includes antithyroid drugs, beta-blockers, radioactive iodine, and surgery.
Thyroid Storm is a severe, life-threatening form of hyperthyroidism requiring urgent medical treatment.
Hypothyroidism is commonly caused by Hashimoto’s thyroiditis. Symptoms include fatigue, weight gain, cold intolerance, and bradycardia. It is diagnosed with elevated TSH and low free T4.
Levothyroxine is the treatment for hypothyroidism, with dosing guided by TSH levels. Myxedema coma is a rare but severe complication.