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 27-year-old G1P0 at 12 weeks estimated gestational age presents for prenatal care. The patient says she has occasional nausea and vomiting and a few episodes of palpitations and diarrhea this last week. Physical examination is unremarkable, except for a heart rate of 145/min. Basic thyroid function tests are shown in the table below. Which of the following additional laboratory tests would be most useful in assessing this patient's condition? Thyroid-stimulating hormone (TSH) 0.28 mIU/L (0.3–4.5 mIU/L) Total T4 12 µg/dL (5.4–11.5 µg/dL)

Thyrotropin receptor antibodies (TRAb)

Total triiodothyronine (T3) levels

Thyroid peroxidase (TPO) antibodies

Thyroxine-binding globulin (TBG) levels

A 49-year-old woman presents to the office because of tremors for 2 months. She says that her hands have been shaking a lot, especially when she feels stressed. In addition, she has been sweating more than usual and has lost 8 kg (17.6 lb) in the last 2 months. She has a past medical history of vitiligo. Her vital signs are a heart rate of 98/min, a respiratory rate of 14/min, a temperature of 37.6°C (99.7°F), and a blood pressure of 115/75 mm Hg. Physical examination shows a fine, bilateral hand tremor and a diffuse goiter. Which of the following hormonal imbalances is most likely present?

Low TSH, high free T4, and high free T3

High TSH, high free T4, and high free T3

High TSH, normal free T4, and normal free T3

Low TSH, normal free T4, and normal free T3

High TSH, low free T4, and low free T3

A 25-year-old man is rushed to the emergency department following a motor vehicle accident. After an initial evaluation, he is found to have bilateral femoral fractures. After surgical fixation of his fractures, he suddenly starts to feel nauseated and becomes agitated. Past medical history is significant for a thyroid disorder. His temperature is 40.0°C (104°F), blood pressure is 165/100 mm Hg, pulse is 170/min and irregularly irregular, and respirations are 20/min. On physical examination, the patient is confused and delirious. Oriented x 0. Laboratory studies are significant for the following: Thyroxine (T4), free 5 ng/dL Thyroid stimulating hormone (TSH) 0.001 mU/L The patient is started on propranolol to control his current symptoms. Which of the following best describes the mechanism of action of this new medication?

Decrease the peripheral conversion of T4 to T3

Inhibition of an underlying autoimmune process

Interference with enterohepatic circulation and recycling of thyroid hormones

Inhibition of release of thyroid hormones

Inhibition of thyroid peroxidase enzyme

A 27-year-old woman presents to her doctor complaining of pain in her neck that radiates to her left ear. The pain has been more or less constant for the last 3 weeks and increases when she chews and swallows. She was in her normal state of health before the pain started. She also mentions that she has been experiencing palpitations, muscle weakness, and increased sweating for the last 2 weeks. Past medical history is significant for a flu-like illness 2 months ago. She currently takes no medication and neither consumes alcohol nor smokes cigarettes. Her pulse is 104/min and irregular with a blood pressure of 140/80 mm Hg. On examination, the physician notices that the patient is restless. There is a presence of fine tremors in both hands. The anterior neck is swollen, warm to the touch, and markedly tender on palpation. Thyroid function tests and a biopsy are ordered. Which of the following deviations from the normal is expected to be seen in her thyroid function tests?

↓ Serum TSH, ↑ Total T4, ↑ Free T4, ↓ I131 Uptake

Normal Serum TSH, ↑ Total T4, Normal Free T4, Normal I131 Uptake

↓ Serum TSH, ↑ Total T4, ↑ Free T4, ↑ I131 Uptake

Normal Serum TSH, ↓ Total T4, Normal Free T4, Normal I131 Uptake

↑ Serum TSH, ↑ Total T4, ↑ Free T4, ↑ I 131 Uptake

Regulation of thyroid function — USMLE Lesson

HPT Axis - The Body's Thermostat

Hypothalamic-Pituitary-Thyroid Axis and hormone regulation

Central Regulation: A classic negative feedback loop maintains thyroid hormone homeostasis, crucial for metabolism, growth, and thermoregulation.

Mechanism: ↑ T3/T4 levels inhibit TRH and TSH release.
Pathology: Disruption at any level (primary, secondary, tertiary) leads to hypo/hyperthyroidism.

⭐ In primary hypothyroidism, ↓T3/T4 leads to a compensatory ↑ in both TRH and TSH. The elevated TSH is the most sensitive initial marker.

Thyroid Kitchen - Cooking Up Hormones

Thyroid hormone regulation and effects

HPT Axis Regulation:

Synthesis Steps:
- Trapping: Iodide ($I^−$) enters follicular cells via Na+/I− symporter (NIS).
- Oxidation & Organification: Thyroid peroxidase (TPO) oxidizes $I^−$ to iodine ($I_2$) and adds it to thyroglobulin (TG) tyrosine residues → MIT & DIT.
- Coupling: TPO couples residues: DIT + DIT → $T_4$; DIT + MIT → $T_3$.
- Release: TSH stimulates proteolysis of TG, releasing $T_4$ and $T_3$ (ratio ~20:1).
Peripheral Conversion: $T_4$ is converted to more potent $T_3$ by 5′-deiodinase.

⭐ Wolff-Chaikoff Effect: High levels of iodide acutely inhibit TPO, ↓ hormone synthesis. This transient effect is used to treat thyroid storm.

On the Road - T4's Big Adventure

Transport: >99% of thyroid hormones are protein-bound in circulation.
- Thyroxine-binding globulin (TBG): Binds ~70%.
- Albumin: Binds ~15-20%.
- Transthyretin (TTR): Binds ~10-15%.
Activation: Thyroxine (T4) is a prohormone.
- Converted to triiodothyronine (T3), the more potent form, in peripheral tissues.
- Enzyme: 5'-deiodinase.
Inactivation: T4 can also be converted to reverse T3 (rT3), which is metabolically inactive.
Free Hormone Hypothesis: Only unbound, free hormone (FT4, FT3) is biologically active.

Pathways of Thyroid Hormone Metabolism

⭐ Estrogen (e.g., in pregnancy or OCP use) ↑ TBG levels, leading to an ↑ in total T4 and T3, but free hormone levels remain normal.

Special Ops - Other Key Regulators

Central Inhibition: Somatostatin, dopamine, and high-dose glucocorticoids can suppress TSH release from the pituitary, acting as a brake on the HPT axis.
TBG Levels: Thyroid-binding globulin levels alter total T4/T3, but free hormone levels remain stable in a healthy individual.
- ↑ TBG: Estrogen (e.g., pregnancy, OCPs) leads to ↑ total T4/T3.
- ↓ TBG: Androgens, anabolic steroids, and protein-losing states (e.g., nephrotic syndrome).

⭐ Pregnancy elevates TBG, increasing total T4/T3. hCG can also weakly stimulate the TSH receptor.

High‑Yield Points - ⚡ Biggest Takeaways

TRH from the hypothalamus stimulates TSH release from the anterior pituitary.

TSH stimulates all steps of thyroid hormone synthesis and release.

Free T3/T4 exert negative feedback on the hypothalamus and pituitary, with T3 being more potent.

Most T3 is formed by peripheral conversion of T4 via 5'-deiodinase.

Wolff-Chaikoff effect: Excess iodide transiently inhibits thyroid peroxidase, decreasing hormone synthesis.

Jod-Basedow phenomenon: Iodide-induced hyperthyroidism in underlying thyroid disease.

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