An investigator is studying a drug that acts on the thyroid hormone pathway. Levels of serum free T3 and T4 in healthy participants are measured before and after administration of the drug. After administration, there is a decrease in the average serum free T3 level, while the average serum free T4 level is increased compared to initial serum studies. Inhibition of which of the following is the most likely mechanism of action of this drug?

Follicular iodotyrosine deiodinase

A 64-year-old man who has not seen a physician in over 20 years presents to your office complaining of recently worsening fatigue and weakness, a decreased appetite, distended abdomen, and easy bruising. His family history is notable for a mother with Hashimoto's thyroiditis, a sister with lupus and a brother with type II diabetes. On further questioning, the patient discloses a history of prior alcoholism as well as intravenous drug use, though he currently only smokes a pack per day of cigarettes. On physical exam, you note the following findings (see Figures A-C) as well as several ecchymoses and telangiectasias. As the patient has not seen a physician in many years, you obtain the following laboratory studies: Leukocyte count: 4,100/mm^3 Hemoglobin: 9.6 g/dL Platelet count: 87,000/mm^3 Prothrombin time (PT): 21.0 seconds International Normalized Ratio (INR): 1.8 Serum: Creatinine: 1.7 mg/dL Total bilirubin: 3.2 mg/dL Aspartate aminotransferase (AST): 225 U/L Alanine aminotransferase (ALT): 103 U/L Alkaline phosphatase: 162 U/L Albumin: 2.6 g/dL Serum thyroxine (T4): 3.1 µg/dL Thyroid-stimulating hormone (TSH): 3.4 µU/mL What is the cause of this patient’s low serum thyroxine?

Decreased liver synthetic function

Transient central hypothyroidism (sick euthyroid syndrome)

Acute hepatitis causing an elevation in thyroxine-binding globulin

Urinary loss of thyroxine-binding globulin due to nephrotic syndrome

An 18-year-old girl comes to the clinic because she is concerned about her weight. She states that she is on her school’s cheerleading team and is upset because she feels she is the “fattest” girl on the team despite her healthy diet. She says that in the last 2 weeks since practice began, she has lost 2 lbs. The patient has bipolar disorder I. Her medications include lithium and a combined oral contraceptive that was recently started by her gynecologist, because “everyone is on it." Her mother has hypothyroidism and is treated with levothyroxine. The patient’s BMI is 23.2 kg/m2. Thyroid function labs are drawn and shown below: Thyroid-stimulating hormone (TSH): 4.0 mIU/L Serum thyroxine (T4): 18 ug/dL Free thyroxine (Free T4): 1.4 ng/dl (normal range: 0.7-1.9 ng/dL) Serum triiodothyronine (T3): 210 ng/dL Free triiodothyronine (T3): 6.0 pg/mL (normal range: 3.0-7.0 pg/mL) Which of the following is the most likely cause of the patient’s abnormal lab values?

Surreptitious use of levothyroxine

A 4-year-old-female presents with a flattened, reddish 2 cm lump located at the base of the tongue. The patient's mother reports her having trouble swallowing, often leading to feeding difficulties. The mother also reports lethargy, constipation, dry skin, and hypothermia. Which of the following is the most appropriate management of this patient’s presentation?

Thyroid hormone replacement therapy with levothyroxine

No treatment is necessary, counsel mother on alternative feeding techniques

Combination therapy of methimazole and Beta-blockers

Thyroid hormone replacements for USMLE Step 2 CK: High-Yield Pharmacology Lessons

Thyroid Physiology - Hormone Factory HQ

Axis: Hypothalamus (TRH) → Anterior Pituitary (TSH) → Thyroid Gland.
Key Enzyme: Thyroid Peroxidase (TPO) drives oxidation & organification of iodide.
Products: Thyroxine ($T_4$) and Triiodothyronine ($T_3$) are synthesized and stored in colloid, bound to thyroglobulin.
Ratio: Thyroid secretes $T_4$ to $T_3$ in a ~20:1 ratio.

⭐ High-Yield: The vast majority (~80%) of active $T_3$ is generated from the peripheral deiodination of $T_4$, primarily in the liver and kidneys.

Thyroid Hormone Synthesis, Storage, and Secretion

The Replacements - Drug Lineup

Levothyroxine (Synthroid, Levoxyl): Synthetic T4.
- Drug of choice for hypothyroidism.
- Metabolized to T3 peripherally; long half-life (~7 days) allows for stable, once-daily dosing.
- Take on an empty stomach, 30-60 minutes before breakfast.
- Steady state achieved in 6-8 weeks.
Liothyronine (Cytomel): Synthetic T3.
- Faster onset, shorter half-life (~1 day).
- Primarily used for myxedema coma (IV) or short-term TSH suppression.
- Associated with ↑ cardiotoxicity risk.
Liotrix: Synthetic T4:T3 in a 4:1 ratio.
Desiccated Thyroid Extract (Armour Thyroid): Porcine-derived.
- Contains T4 & T3; inconsistent potency and antigenicity concerns.

DTE vs T4 on Thyroid Hormones and Heart Rate

⭐ Levothyroxine's long half-life is a key clinical advantage. It ensures a stable reservoir, meaning a missed dose doesn't cause significant fluctuation, making it forgiving for patients and reliable for maintaining euthyroid states.

Dosing & Monitoring - The Titration Game

Initial Dose (Levothyroxine):
- Standard: ~$1.6$ mcg/kg/day (ideal body weight).
- Elderly / Cardiac Disease: Start low, go slow. Initial dose 12.5-25 mcg/day to avoid cardiac stress.
Titration & Monitoring:
- Primary Goal: Normalize TSH to 0.4-4.0 mIU/L.
- Check TSH 4-6 weeks after any dose change.
- Adjust dose by 12.5-25 mcg increments.
  - High TSH (Hypo): ↑ Levothyroxine dose.
  - Low TSH (Hyper): ↓ Levothyroxine dose.
Special Considerations:
- Pregnancy: Often requires a dose ↑ of 30-50%.

⭐ Levothyroxine's long half-life (~7 days) means steady state takes over a month. This is why TSH is re-checked 4-6 weeks post-dose adjustment, not earlier.

Levothyroxine Dosing and TSH Monitoring by Patient Type

Adverse Effects & Interactions - Too Much of a Good Thing

Essentially iatrogenic hyperthyroidism. Symptoms mirror thyrotoxicosis.

ECG: Atrial Fibrillation in Thyrotoxicosis

Cardiovascular: Tachycardia, palpitations, angina, arrhythmias (esp. atrial fibrillation in elderly).
Neurological: Nervousness, anxiety, tremor, insomnia.
Metabolic/General: ↑ aBMR, weight loss despite ↑ appetite, heat intolerance, sweating.
Musculoskeletal: Muscle weakness, ↑ bone turnover leading to osteoporosis with long-term overuse.

⭐> High-Yield: Over-replacement, especially in older adults, significantly increases the risk of atrial fibrillation and accelerated osteoporosis.

Drug Interactions:

↓ Absorption: Bile acid sequestrants (cholestyramine), iron, calcium supplements, PPIs.
↑ Metabolism: Rifampin, Phenytoin, Carbamazepine induce CYP450 enzymes.

High‑Yield Points - ⚡ Biggest Takeaways

Levothyroxine (T4) is the mainstay treatment for hypothyroidism, acting as a prodrug converted to active T3 peripherally.

Liothyronine (T3) has a faster onset and is reserved for severe, acute conditions like myxedema coma.

Monitor therapy with TSH levels; the goal is to bring TSH into the normal range.

Adverse effects are symptoms of iatrogenic hyperthyroidism: palpitations, anxiety, and weight loss.

Take on an empty stomach; absorption is impaired by food, iron, and calcium.

Increases warfarin's anticoagulant effect.

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