A 34-year-old woman presents to the clinic with a 2-month history of unintentional weight … — USMLE Step 3

A 34-year-old woman presents to the clinic with a 2-month history of unintentional weight loss, frequent palpitations, and heat intolerance. On physical examination, her heart rate is $112/\text{min}$ , and she has a fine resting tremor. Neck examination reveals a diffusely enlarged, non-tender thyroid gland. Laboratory evaluation demonstrates a thyroid-stimulating hormone (TSH) level of $0.01 \text{ }\mu\text{U/mL}$ (reference: $0.4-4.0 \text{ }\mu\text{U/mL}$ ) and a free thyroxine ( $T_4$ ) level of $3.2 \text{ ng/dL}$ (reference: $0.9-1.7 \text{ ng/dL}$ ). She is diagnosed with Graves disease and prescribed methimazole. To understand the cellular mechanism of this patient's disease and her new medication, the physician reviews the physiological sequence of thyroid hormone synthesis. Arrange the following steps of normal thyroid hormone synthesis and release in the correct chronological order, beginning with the initial uptake of the raw material from the bloodstream.

1Basolateral transport of iodide into the follicular cell via the $\text{Na}^+/\text{I}^-$ symporter (NIS)
2Apical transport of iodide into the colloid space mediated by pendrin
3Thyroid peroxidase-mediated oxidation of iodide to highly reactive iodine
4Organification of iodine onto specific tyrosine residues of thyroglobulin
5Coupling of iodotyrosine molecules to form triiodothyronine ( $T_3$ ) and thyroxine ( $T_4$ )
6Endocytosis of colloid and lysosomal proteolysis to release active hormone

Answer

The correct sequence is: Basolateral iodide transport (NIS) -> Apical iodide transport (Pendrin) -> Oxidation (TPO) -> Organification -> Coupling -> Endocytosis/Proteolysis.

Thyroid hormone synthesis begins with the active concentration of inorganic iodide from the blood into the follicular cell via the basolateral sodium-iodide symporter (NIS). Iodide then diffuses across the cell and is transported into the follicular lumen (colloid) via pendrin on the apical membrane. At the apical-colloid interface, thyroid peroxidase (TPO) oxidizes iodide to a reactive iodine species. This iodine is immediately organified onto tyrosine residues of the large thyroglobulin protein, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). TPO then catalyzes the coupling of these molecules to form

T_3

and

T_4

, which remain attached to thyroglobulin in the colloid. Upon stimulation (e.g., by TSH or TSH-receptor antibodies in Graves disease), the cell endocytoses the colloid. Lysosomes fuse with these vesicles and proteolytically cleave the thyroglobulin, releasing free

T_3

and

T_4

into the bloodstream.

Step-by-Step Solution

Identify the starting point of the pathway.

Basolateral transport of iodide.

Iodide must first be extracted from the capillary blood into the thyroid follicular cell via the

\text{Na}^+/\text{I}^-

symporter (NIS) on the basolateral membrane.

Determine the next compartment the substrate must reach.

Apical transport into the colloid.

Hormone synthesis occurs in the follicular lumen. Iodide exits the apical membrane via the pendrin transporter.

Sequence the three enzymatic steps catalyzed by Thyroid Peroxidase (TPO).

Oxidation, then organification, then coupling.

Iodide is oxidized to iodine, which is then organified onto thyroglobulin tyrosine residues (forming MIT/DIT). Finally, MIT and DIT are coupled to form

T_3

and

T_4

on the thyroglobulin backbone.

Identify the final step for hormone release.

Endocytosis and lysosomal proteolysis.

The colloid containing iodinated thyroglobulin is endocytosed back into the follicular cell, where lysosomes cleave the active

T_3

and

T_4

for basolateral secretion.

Key Concept

Thyroid hormone synthesis requires a highly coordinated sequence of membrane transport (NIS, pendrin) and enzymatic reactions catalyzed by thyroid peroxidase (oxidation, organification, coupling) occurring at the apical membrane and within the colloid space.

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