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 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

An investigator studying hormone synthesis and transport uses immunocytochemical techniques to localize a carrier protein in the central nervous system of an experimental animal. The investigator finds that this protein is synthesized together with a specific hormone from a composite precursor. The protein is involved in the transport of the hormone from the supraoptic and paraventricular nuclei to its destination. The hormone transported by these carrier proteins is most likely responsible for which of the following functions?

Upregulation of renal aquaporin-2 channels

Stimulation of thyroglobulin cleavage

Hyperplasia of the adrenal zona fasciculata

Increased insulin-like growth factor 1 production

Maturation of primordial germ cells

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 55-year-old woman has a total thyroidectomy for papillary thyroid carcinoma. She complains of tingling around the mouth 11 hours after the operation. Her condition rapidly deteriorates with difficulty breathing and chest tightness. Which of the following best represent the signaling pathway of the deficient hormone responsible for this patient’s symptoms?

Cyclic adenosine monophosphate (cAMP)

Cyclic guanosine monophosphate (cGMP)

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

Thyroid hormone transport and action for USMLE Step 3: High-Yield Physiology Lessons

Thyroid Hormone Synthesis - The Iodine Job

Thyroid Hormone Synthesis and Metabolism

Iodide Trapping: Dietary iodide is actively transported into follicular cells via the Na+/I- symporter (NIS).
Key Enzyme: Thyroid peroxidase (TPO) on the apical membrane manages:
- Oxidation: Iodide (I⁻) to iodine (I₂).
- Organification: Iodine added to thyroglobulin (TG) to form MIT & DIT.
- Coupling: MIT + DIT → T₃; DIT + DIT → T₄.

⭐ Wolff-Chaikoff Effect: Excess iodide temporarily inhibits TPO, ↓ hormone synthesis. This is a protective autoregulatory effect. "Escape" from this effect occurs via downregulation of the NIS.

Drug Targets:
- Perchlorate, Thiocyanate: Inhibit NIS.
- Propylthiouracil (PTU), Methimazole: Inhibit TPO.

Bloodstream Convoy - The Great Carrier Heist

Transport: >99% of T3/T4 is protein-bound in blood; only free hormone is active.
- Thyroxine-Binding Globulin (TBG): Binds most T4 & T3. Levels ↑ in pregnancy (estrogen), ↓ in liver failure, nephrotic syndrome.
- Other carriers: Albumin, Transthyretin.
Action:
- T4 is a prohormone, converted to T3 (more potent) in peripheral tissues by 5'-deiodinase.
- T3 acts via nuclear receptors to regulate gene transcription.

⭐ High-Yield: Drugs like salicylates and phenytoin can displace thyroid hormone from TBG, transiently increasing free hormone levels.

Peripheral Action - T4's Big Makeover

Prohormone to Active Form: Thyroxine ($T_4$) is the main circulating hormone, but Triiodothyronine ($T_3$) is 3-4x more potent. $T_4$ functions as a prohormone, converted to $T_3$ in target tissues.
Key Enzyme: Deiodinase:
- 5'-deiodinase (Types I & II) converts $T_4 \rightarrow T_3$ (activation). Found in liver, kidney, brain, pituitary.
- 5-deiodinase (Type III) converts $T_4 \rightarrow$ reverse $T_3$ ($rT_3$, inactive) and $T_3 \rightarrow T_2$ (inactivation).
Cellular Action:
- $T_3$ enters the cell and binds to high-affinity nuclear receptors (TRα, TRβ).
- This hormone-receptor complex acts as a transcription factor, binding to Thyroid Response Elements (TREs) on DNA to alter gene expression and metabolic rate.

Thyroid hormone action on nuclear receptor

⭐ High-Yield Fact: Drugs like propylthiouracil (PTU), propranolol, amiodarone, and corticosteroids can ↓ the peripheral conversion of $T_4$ to $T_3$. This is a key mechanism for rapidly controlling symptoms in thyroid storm.

HPT Axis - The Control Tower

Thyroid Hormone Transport and Action Diagram

Hypothalamus: Releases Thyrotropin-Releasing Hormone (TRH) in a pulsatile fashion.
Anterior Pituitary: TRH stimulates thyrotrophs to release Thyroid-Stimulating Hormone (TSH).
Thyroid Gland: TSH acts on follicular cells (via Gs pathway) to synthesize and release T4 and T3.

Negative Feedback: Free T3/T4 inhibit TRH and TSH secretion, maintaining hormonal balance.

⭐ Inhibition: TSH release is also inhibited by somatostatin, dopamine, and high levels of glucocorticoids. This is a key regulatory point outside of direct thyroid feedback.

High‑Yield Points - ⚡ Biggest Takeaways

Most thyroid hormone circulates bound to thyroxine-binding globulin (TBG); only the free hormone is active.

Peripheral conversion of T4 to T3 by 5'-deiodinase is a key activation step; T3 is more potent.

Thyroid hormones act via intracellular nuclear receptors, directly influencing gene transcription and protein synthesis.

They are critical for regulating basal metabolic rate (BMR) and CNS maturation.

They upregulate β-adrenergic receptors, increasing sensitivity to catecholamines.

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