A 27-year-old woman comes to the physician because of a 2-month history of palpitations, diaphoresis, and a 5-kg (11-lb) weight loss. Her pulse is 101/min and blood pressure is 141/84 mm Hg. Physical examination shows a fine tremor when the fingers are outstretched. After confirmation of the diagnosis, treatment is begun with an antithyroid medication. The physician emphasizes the need for adequate contraception because of the increased risk of severe fetal malformations associated with the use of this medication, which is why its use is discouraged in the first trimester of pregnancy. Which of the following best describes the mechanism of action of this drug?

Inhibition of iodide ion oxidation

Inhibition of thyroid hormone release

Inhibition of peripheral conversion of T4 to T3

Decreased iodide uptake by follicular cells

Suppression of thyroid-stimulating hormone release

A patient with Graves' disease is treated with thiocyanate (a historical antithyroid agent). Thiocyanate helps reduce thyroid hormone production by:

Inhibiting iodide follicular uptake

Inhibiting beta-adrenergic receptors

A 19-year-old woman, accompanied by her parents, presents after a one-week history of abnormal behavior, delusions, and unusual aggression. She denies fever, seizures or illicit drug use. Family history is negative for psychiatric illnesses. She was started on risperidone and sent home with her parents. Three days later, she is brought to the emergency department with fever and confusion. She is not verbally responsive. At the hospital, her temperature is 39.8°C (103.6°F), the blood pressure is 100/60 mm Hg, the pulse rate is 102/min, and the respiratory rate is 16/min. She is extremely diaphoretic and appears stiff. She has spontaneous eye-opening but she is not verbally responsive and she is not following commands. Laboratory studies show: Sodium 142 mmol/L Potassium 5.0 mmol/L Creatinine 1.8 mg/dl Calcium 10.4 mg/dl Creatine kinase 9800 U/L White blood cells 14,500/mm3 Hemoglobin 12.9 g/dl Platelets 175,000/mm3 Urinalysis shows protein 1+, hemoglobin 3+ with occasional leukocytes and no red blood casts. What is the best first step in the management of this condition?

Switch risperidone to clozapine

A 25-year-old female with a history of childhood asthma presents to clinic complaining of a three month history of frequent, loose stools. She currently has three to four bowel movements per day, and she believes that these episodes have been getting worse and are associated with mild abdominal pain. She also endorses seeing red blood on the toilet tissue. On further questioning, she also endorses occasional palpitations over the past few months. She denies fevers, chills, headache, blurry vision, cough, shortness of breath, wheezing, nausea, or vomiting. She describes her mood as slightly irritable and she has been sleeping poorly. A review of her medical chart reveals a six pound weight loss since her visit six months ago, but she says her appetite has been normal. The patient denies any recent illness or travel. She is a non-smoker. Her only current medication is an oral contraceptive pill. Her temperature is 37°C (98.6°F), pulse is 104/min, blood pressure is 95/65 mmHg, respirations are 16/min, and oxygen saturation is 99% on room air. On physical exam, the physician notes that her thyroid gland appears symmetrically enlarged but is non-tender to palpation. Upon auscultation there is an audible thyroid bruit. Her cranial nerve is normal and ocular exam reveals exophthalmos. Her abdomen is soft and non-tender to palpation. Deep tendon reflexes are 3+ throughout. Lab results are as follows: Serum: Na+: 140 mEq/L K+: 4.1 mEq/L Cl-: 104 mEq/L HCO3-: 26 mEq/L BUN: 18 mg/dL Creatinine 0.9 mg/dL Hemoglobin: 14.0 g/dL Leukocyte count: 7,400/mm^3 Platelet count 450,000/mm^3 TSH & Free T4: pending A pregnancy test is negative. The patient is started on propranolol for symptomatic relief. What is the most likely best next step in management for this patient?

Thyroid scintigraphy with I-123

Antithyroid medications for USMLE Step 1: High-Yield Pharmacology Lessons

Thyroid Hormone Synthesis - The Hormone Factory

Thyroid Hormone Synthesis in Follicular Cell Diagram

Iodide Trapping: Na+/I- symporter (NIS) actively transports iodide into the follicular cell.
Oxidation & Organification: Thyroid peroxidase (TPO) oxidizes iodide to iodine and incorporates it into thyroglobulin (Tg) tyrosine residues, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT).
Coupling: TPO couples MIT and DIT to form T3 (MIT + DIT) and T4 (DIT + DIT).
Storage & Release: Hormones are stored as colloid. TSH stimulation triggers release.

⭐ Wolff-Chaikoff Effect: High levels of intracellular iodide paradoxically inhibit TPO, temporarily shutting down hormone synthesis. This is a protective autoregulatory mechanism.

Thionamides (MMI, PTU) - Production Blockers

Mechanism: Inhibit thyroid peroxidase (TPO), blocking iodine organification and coupling of iodotyrosines. This halts new thyroid hormone synthesis.

Propylthiouracil (PTU):
- Also blocks peripheral conversion of T4 → T3.
- 📌 Use in Pregnancy (1st trimester) & Thyroid storm Urgencies.
Methimazole (MMI):
- Preferred agent (longer half-life), but is a known teratogen (aplasia cutis).
Adverse Effects:
- Maculopapular rash (most common).
- ⚠️ Agranulocytosis (fever, sore throat).
- Hepatotoxicity (severe with PTU).

⭐ Patients on thionamides with fever or pharyngitis require an urgent CBC to exclude agranulocytosis, a potentially fatal complication.

Iodide & Radioiodine - Sabotage & Destruction

Iodide (Lugol’s solution, Potassium Iodide - KI)
- Mechanism: High levels inhibit organification (Wolff-Chaikoff effect) & block T4/T3 release.
- Use: Rapidly ↓ hormone release in thyroid storm or pre-operatively for thyroidectomy.
- Onset: Fast (2-7 days), but transient effect (escape phenomenon).
Radioactive Iodine ($^{131}$I)
- Mechanism: Emits β-particles, causing selective destruction of follicular cells.
- Use: Definitive oral treatment for Graves' disease & toxic nodules.
- Adverse Effects: Permanent hypothyroidism (common), potential worsening of ophthalmopathy.

⭐ Contraindication: Absolutely contraindicated in pregnancy and breastfeeding as it ablates the fetal thyroid. A pregnancy test is mandatory before administration.

Thyroid Scan: Diagnostic vs. Post-Radioactive Iodine

Anion Inhibitors & Adjuvants - Gate Crashers & Symptom Tamers

Anion Inhibitors: Perchlorate ($ClO_4^-$), Pertechnetate ($TcO_4^-$)
- MOA: Competitively inhibit the Na+/I- Symporter (NIS), blocking iodide uptake.
- ⚠️ Limited use due to aplastic anemia risk.
Adjuvants:
- Beta-blockers: Control adrenergic symptoms (tremor, palpitations).
- Iodinated Contrast Media: Inhibit peripheral T4→T3 conversion.

⭐ Propranolol offers a dual benefit: rapid relief of sympathetic symptoms plus decreased peripheral conversion of T4 to T3.

High-Yield Points - ⚡ Biggest Takeaways

Thionamides (methimazole, PTU) inhibit thyroid peroxidase, blocking iodine organification and coupling reactions.

PTU also blocks the peripheral conversion of T4 to T3, making it useful in thyroid storm.

Methimazole is generally preferred but is teratogenic (aplasia cutis); use PTU in the 1st trimester of pregnancy.

The most feared side effect of thionamides is agranulocytosis; monitor for fever and sore throat.

Radioactive iodine (¹³¹I) ablates the thyroid but is contraindicated in pregnancy.

Iodide solutions (Lugol's) trigger the Wolff-Chaikoff effect, acutely inhibiting thyroid hormone release.

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Antithyroid medications