A 23-year-old primigravida presents to her physician’s office at 12 weeks gestation complaining of increased sweating and palpitations for the last week. She does not have edema or dyspnea, and had no pre-existing illnesses. The patient says that the symptoms started a few days after several episodes of vomiting. She managed the vomiting at home and yesterday the vomiting stopped, but the symptoms she presents with are persistent. The pre-pregnancy weight was 54 kg (119 lb). The current weight is 55 kg (121 lb). The vital signs are as follows: blood pressure 130/85 mm Hg, heart rate 113/min, respiratory rate 15/min, and temperature 37.0℃ (98.6℉). The physical examination is significant for diaphoresis, an irregular heartbeat, and a fine resting tremor of the hands. The neck is not enlarged and the thyroid gland is not palpable. The ECG shows sinus tachyarrhythmia. The thyroid panel is as follows: Thyroid stimulating hormone (TSH) < 0.1 mU/L Total T4 178 nmol/L Free T4 31 pmol/L Which of the following is indicated?

Ensure proper hydration and prescribe a beta-blocker

Schedule a subtotal thyroidectomy

A 28-year-old G1P0 woman comes to the emergency department complaining that her water just broke. She reports irregular prenatal care due to her erratic schedule. She is also unsure of her gestational age but claims that her belly began to show shortly after she received her thyroidectomy for her Graves disease about 9 months ago. She denies any known fevers, chills, abnormal vaginal discharge/bleeding, or sexually transmitted infections. She develops frequent and regular contractions and subsequently goes into active labor. A fetus was later vaginally delivered with a fetal heart rate of 180 bpm. A neonatal physical examination demonstrates a lack of a sagittal cranial suture and an APGAR score of 8 and 8, at 1 and 5 minutes respectively. What findings would you expect in the baby?

High levels of free T4 and total T3

High thyroid-stimulating hormone

A 26-year-old G1P0 presents to her first obstetric visit after having a positive urine pregnancy test at home. Her last menstrual period was 9 weeks ago. She has no past medical history, but her mother has rheumatoid arthritis. The patient states that for several weeks, she has felt especially warm, even when her co-workers do not, and had muscle weakness. She also complains of mood swings and fatigue. At this visit, her temperature is 99.0°F (37.2°C), blood pressure is 140/81 mmHg, pulse is 106/min, and respirations are 17/min. Physical exam is notable for 3+ deep tendon reflexes bilaterally and 4/5 strength in both hips and shoulders. Ultrasound confirms the presence of a heart beat and shows a crown rump length that is consistent with a gestational age of 9 weeks and 3 days. Which of the following is the best therapy for this patient?

Radioactive thyroid ablation (I-131)

A 25-year-old woman presents to her physician with a missed mense and occasional morning nausea. Her menstrual cycles have previously been normal and on time. She has hypothyroidism resulting from Hashimoto thyroiditis diagnosed 2 years ago. She receives levothyroxine (50 mcg daily) and is euthyroid. She does not take any other medications, including birth control pills. At the time of presentation, her vital signs are as follows: blood pressure 120/80 mm Hg, heart rate 68/min, respiratory rate 12/min, and temperature 36.5℃ (97.7℉). The physical examination shows slight breast engorgement and nipple hyperpigmentation. The gynecologic examination reveals cervical softening and increased mobility. The uterus is enlarged. There are no adnexal masses. The thyroid panel is as follows: Thyroid stimulating hormone (TSH) 3.41 mU/L Total T4 111 nmol/L Free T4 20 pmol/L Which of the following adjustments should be made to the patient’s therapy?

Increase levothyroxine dosage by 20%–30%

Decrease levothyroxine dosage by 30%

The patient is euthyroid, so no adjustments should be made

Increase levothyroxine dosage by 5% each week up to 50%

A 25-year-old woman, gravida 2, para 1, at 25 weeks' gestation comes to the emergency department because of a 1-day history of fever and right-sided flank pain. During this period, she also had chills, nausea, vomiting, and burning on urination. Her last prenatal visit was 10 weeks ago. Pregnancy and delivery of her first child were uncomplicated. Her temperature is 39°C (102.2°F), pulse is 110/min, respirations are 20/min, and blood pressure is 110/70 mm Hg. Physical examination shows costovertebral angle tenderness on the right. The abdomen is soft and nontender, and no contractions are felt. Pelvic examination shows a uterus consistent in size with a 25-week gestation. Fetal heart rate is 170/min. Laboratory studies show: Leukocyte count 15,000/mm3 Urine Nitrite 2+ Protein 1+ Blood 1+ RBC 5/hpf WBC 500/hpf Blood and urine samples are obtained for culture and drug sensitivity. Which of the following is the most appropriate next step in management?

Inpatient treatment with intravenous ceftriaxone

Outpatient treatment with oral ciprofloxacin

Inpatient treatment with intravenous ampicillin and gentamicin

Admit the patient and request an emergent obstetrical consult

A 36-year-old primigravida presents to her obstetrician for antenatal care. She is at 24 weeks of gestation and does not have any current complaint except for occasional leg cramps. She does not smoke or drink alcohol. Family history is irrelevant. Her temperature is 36.9°C (98.42°F), blood pressure is 100/60 mm Hg, and pulse of 95/minute. Her body mass index is 21 kg/m² (46 pounds/m²). Physical examination reveals a palpable uterus above the umbilicus with no other abnormalities. Which of the following screening tests is suitable for this patient?

Oral glucose tolerance test for gestational diabetes mellitus

Fasting and random glucose testing for gestational diabetes mellitus

HbA1C for gestational diabetes mellitus

Complete blood count for iron deficiency anemia

Wet mount microscopy of vaginal secretions for bacterial vaginosis

A 28-year-old primigravid woman comes to the emergency department because of a 12-hour history of lower abdominal pain and vaginal bleeding. She also had nausea and fatigue for the past 3 weeks. Her last menstrual period was 8 weeks ago. Prior to that, her menses occurred regularly at 30-day intervals and lasted for 4 days. There is no history of medical illness, and she takes no medications. Her temperature is 37°C (98.6°F), pulse is 95/min, and blood pressure is 100/70 mm Hg. Pelvic examination is painful and shows a uterus consistent in size with a 13-week gestation. A urine pregnancy test is positive. β-HCG level is 106,000 mIU/mL (N < 5 mIU/mL). Transvaginal ultrasonography shows unclear, amorphous fetal parts and a large placenta with multiple cystic spaces. Which of the following is the most likely cause of this patient's condition?

Trophoblastic proliferation with chorionic villi distention

Placenta implantation into myometrium

Embryonic death with cervical dilation

Malpositioned placenta overlying the cervix

Malignant transformation of trophoblastic tissue

Thyroid disorders in pregnancy for USMLE Step 1: High-Yield OB/GYN Lessons

Thyroid Physiology in Pregnancy - Hormone High Jinks

Estrogen: ↑ Estrogen → ↑ hepatic synthesis of Thyroxine-Binding Globulin (TBG).
- Binds more thyroid hormone, leading to ↑ Total T4 (TT4) and Total T3 (TT3).
- Free T4/T3 levels remain relatively stable.
hCG: The α-subunit of hCG is similar to TSH, weakly stimulating the TSH receptor.
- Results in ↑ T4/T3 production.
- Causes a physiological ↓ in TSH, especially in the 1st trimester.

Thyroid hormone changes and fetal dependency in pregnancy

⭐ High first-trimester hCG levels can cause transient gestational thyrotoxicosis and are linked to hyperemesis gravidarum; TSH will be appropriately suppressed.

Hypothyroidism in Pregnancy - Low & Slow

Etiology: Most common cause is Hashimoto's thyroiditis (anti-TPO antibodies).
Maternal Risks: Preeclampsia, anemia, miscarriage, postpartum hemorrhage.
Fetal/Neonatal Risks: Impaired neurodevelopment (cretinism), preterm birth, low birth weight.
Management: Treat with levothyroxine to maintain TSH < 2.5 mU/L. Monitor TSH every 4-6 weeks during pregnancy and adjust dose as needed.

⭐ Maternal thyroxine (T4) is crucial for fetal brain development before the fetal thyroid becomes functional around 12 weeks gestation.

Maternal-fetal thyroid hormone and iodine metabolism

Hyperthyroidism in Pregnancy - High & Hasty

Etiology: Most commonly Graves' disease (~95%). Can also be hCG-mediated (molar pregnancy, multiple gestations) as hCG α-subunit mimics TSH.
Diagnosis: ↓TSH, ↑Free T4/T3. TSH-receptor antibodies (TRAb) confirm Graves'.
Maternal Risks: Preeclampsia, heart failure, thyroid storm.
Fetal Risks: Goiter, tachycardia, IUGR, hydrops fetalis, preterm birth.

⭐ Methimazole (MMI) use in the first trimester is associated with a specific pattern of birth defects known as methimazole embryopathy, most notably aplasia cutis congenita.

Aplasia cutis congenita from methimazole exposure

Management:
- 1st Trimester: Propylthiouracil (PTU). 📌 PTU for Primary trimester.
- 2nd/3rd Trimester: Switch to Methimazole (MMI) to avoid PTU's hepatotoxicity risk.

Postpartum Thyroiditis - The Aftermath

Autoimmune thyroiditis occurring within 1 year postpartum, often revealing underlying Hashimoto's.
Presents with a triphasic course: transient hyperthyroidism, then hypothyroidism, followed by recovery.
Diagnosis: Based on clinical picture and positive anti-TPO antibodies. Radioiodine uptake is low.

⭐ There is a 20-40% risk of developing permanent hypothyroidism and a high risk of recurrence in future pregnancies.

Management: Symptomatic. Use β-blockers for thyrotoxicosis and Levothyroxine for symptomatic hypothyroidism.

High-Yield Points - ⚡ Biggest Takeaways

Maternal hypothyroidism is a major risk for impaired fetal neurodevelopment and cretinism.

hCG can stimulate TSH receptors, leading to gestational transient thyrotoxicosis in the 1st trimester.

Graves' disease is the most common cause of hyperthyroidism; TSH-receptor antibodies cross the placenta.

Treat hyperthyroidism with propylthiouracil (PTU) in the 1st trimester, then switch to methimazole.

PTU has a risk of maternal hepatotoxicity; methimazole is linked to aplasia cutis.

Thyroid storm is an obstetric emergency requiring beta-blockers, PTU, and steroids.

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