Practice 10 structured short- and long-answer medical questions on Thyroid Hormone Physiology and Pathology. Review model answers for exam revision.
Answer: Thyroid hormone synthesis is dependent on several key factors: * **Iodine:** Essential for production. It is actively transported into follicular cells via the sodium-iodide symporter (NIS) and incorporated into tyrosine residues. * **Thyroid-Stimulating Hormone (TSH):** Released by the anterior pituitary; it regulates iodine uptake, thyroglobulin synthesis, and hormone release. * **Thyroglobulin:** A glycoprotein precursor synthesized in follicular cells. It provides the tyrosine residues for iodination. * **Thyroid Peroxidase (TPO):** An enzyme that catalyzes the oxidation of iodide, iodination of tyrosine (forming MIT and DIT), and the coupling of these residues to form T3 and T4. ### Clinical Significance of Reverse T3 (rT3) Reverse T3 is an inactive isomer of T3 produced by the deiodination of T4. Its significance includes: * **Metabolic Regulation:** It acts as a regulatory mechanism to lower thyroid activity without changing T4 production. * **Non-Thyroidal Illness Syndrome (NTIS):** During starvation, severe stress, or systemic illness, the body increases conversion of T4 to rT3 instead of active T3 to conserve energy. * **Clinical Marker:** Elevated rT3 levels are seen in chronic stress, liver disease, and severe malnutrition. ### The Hypothalamus-Pituitary-Thyroid (HPT) Axis
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Answer: ### Molecular Mechanism of Action * **Transport:** T3 and T4 enter cells via transporters like MCT8. T4 is converted to T3 by intracellular deiodinases. * **Nuclear Binding:** T3 binds to Thyroid Hormone Receptors (TR) in the nucleus. These receptors often form heterodimers with Retinoid X Receptors (RXR) on Thyroid Response Elements (TRE) of DNA. * **Transcription:** Binding causes a conformational change, displacing corepressors and recruiting coactivators, leading to gene transcription. * **Effects:** Increases Basal Metabolic Rate (BMR), stimulates gluconeogenesis, promotes lipid mobilization, and regulates thermogenesis. ### Hypothyroidism **Five Primary Causes:**
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Answer: ### Management of Radioactive Iodine (131I) Exposure * **Potassium Iodide (KI):** Administered to saturate the thyroid with stable iodine, blocking the uptake of radioactive isotopes. * **Levothyroxine:** Hormone replacement therapy if the thyroid is ablated or damaged. * **Monitoring:** Regular TSH/T4 screening and ultrasound monitoring for radiation-induced nodules or thyroid cancer. ### Pharmacology Summary * **Thionamides:** Propylthiouracil (PTU), Methimazole, Carbimazole (Inhibit TPO and hormone synthesis). * **Anion Inhibitors:** Thiocyanate (Inhibit iodide uptake). * **Iodides:** High doses inhibit hormone release (Wolff-Chaikoff effect). ### Case Studies **Case Study 1** * **Presentation:** 25F, 4 months postpartum, fatigue, irregular menses, family history of Hashimoto's. TSH 0.9 (Normal), Free T4 0.8 (Low-Normal). * **Next Step:** Perform thyroid antibody testing (Anti-TPO, Anti-Tg) to evaluate for Postpartum Thyroiditis. **Case Study 2** * **Presentation:** 39F, history of galactorrhea/amenorrhea, stopped bromocriptine, now has headaches, prolactin 60.5 ng/mL, and a 2.4 cm sellar mass with superotemporal field cut. * **Diagnosis:** (A) Prolactinoma. The clinical triad of galactorrhea, elevated prolactin, and a macroadenoma causing optic chiasm compression is classic.