Confusable diagnoses · PANCE / PANRE

Hyperthyroidism vs Hypothyroidism

Written & clinically reviewed by Ryan Busch, PA-C · Updated August 2026 · Report an error

Hyperthyroidism and Hypothyroidism are easy to mix up on the boards. Here's a side-by-side comparison — presentation, workup, imaging, and first-line treatment — drawn from our full outlines.

Hyperthyroidism vs Hypothyroidism at a glance

  • Hyperthyroidism: Autoimmune diffuse goiter driven by TSH-receptor stimulating antibodies; most common cause of overt hyperthyroidism.
  • Hypothyroidism: Autoimmune chronic lymphocytic thyroiditis; most common cause of hypothyroidism in iodine-replete regions.

Try two board-style questions on Hyperthyroidism vs Hypothyroidism

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Question 1EndocrineMedium
A 30-year-old woman at 10 weeks gestation has nausea, palpitations, and a diffusely enlarged thyroid. Free T4 is elevated, TSH is suppressed, and thyroid receptor antibody (TRAb) is positive at 1:320. She has no prior thyroid history. Which of the following antithyroid medications is most appropriate at this stage of pregnancy?
  • ALithium carbonate
  • BPropylthiouracil
  • CMethimazole
  • DPotassium iodide
Reveal answer & full explanation
Correct answer: B — Propylthiouracil
  • ALithium carbonate
  • BPropylthiouracil✓
  • CMethimazole
  • DPotassium iodide

Why Propylthiouracil is correct

  • Elevated free T4, suppressed TSH, a diffuse goiter, and a positive TRAb in a 10-week-pregnant patient establish Graves disease as the cause of gestational hyperthyroidism
  • During the first trimester (organogenesis, roughly weeks 6-10), the American Thyroid Association recommends propylthiouracil (PTU) over methimazole because methimazole is associated with a characteristic embryopathy (aplasia cutis, choanal/esophageal atresia, omphalocele)
  • Untreated Graves disease in pregnancy raises the risk of fetal loss, preterm delivery, and maternal heart failure, so antithyroid therapy is required rather than observation
  • Target the free T4 at or just above the upper limit of normal using the lowest effective dose

Why the others are wrong

  • Lithium carbonate — inhibits thyroid hormone release and is used off-label only when thionamides and iodine cannot be given; it is not an ATA-recommended antithyroid drug, and first-trimester exposure carries a risk of Ebstein anomaly (confused-with drug-induced thyroid dysfunction)
  • Methimazole — effective but teratogenic during first-trimester organogenesis; it becomes the preferred agent only in the second and third trimesters, when PTU hepatotoxicity outweighs its benefit (right-drug-wrong-trimester)
  • Potassium iodide — iodide acutely blocks hormone release and is reserved for thyroid storm or short-term preoperative preparation; sustained use crosses the placenta and can cause fetal goiter and hypothyroidism (right-drug-wrong-setting)

Additional high-yield points

  • Trimester-specific switch: change PTU to methimazole after the first trimester because prolonged PTU use carries rare but serious hepatotoxicity
  • TRAb crosses the placenta; titers above 3x the upper limit of normal predict fetal/neonatal hyperthyroidism, so monitor fetal heart rate and growth and check neonatal thyroid function at delivery
Question 2EndocrineMedium
A 32-year-old woman with primary hypothyroidism on a stable levothyroxine dose learns she is 8 weeks pregnant. Which of the following best describes how her levothyroxine requirement typically changes in pregnancy?
  • ANo dose adjustment is needed
  • BUnpredictable changes requiring T3 supplementation
  • CIncrease by 25-50%
  • DDecrease by 10-20%
Reveal answer & full explanation
Correct answer: C — Increase by 25-50%
  • ANo dose adjustment is needed
  • BUnpredictable changes requiring T3 supplementation
  • CIncrease by 25-50%✓
  • DDecrease by 10-20%

Why Increase by 25-50% is correct

  • Thyroid hormone requirements rise early in pregnancy, driven by the estrogen-mediated increase in thyroxine-binding globulin, transplacental T4 transfer, and increased placental type 3 deiodinase activity
  • Per the American Thyroid Association, women with treated hypothyroidism need roughly a 25-50% increase in levothyroxine, often achieved by adding two extra doses per week (a 'nine doses per week' strategy) as soon as pregnancy is confirmed
  • Adequate replacement is critical in the first trimester, before the fetal thyroid is functional, to prevent miscarriage, preterm birth, and impaired fetal neurodevelopment

Why the others are wrong

  • No dose adjustment is needed — this ignores the predictable rise in TBG and metabolic demand and risks maternal/fetal hypothyroidism (premature closure)
  • Unpredictable changes requiring T3 supplementation — the change is predictable (an increase), and levothyroxine (T4) monotherapy remains standard; T3 is not recommended in pregnancy (confused-with combination therapy)
  • Decrease by 10-20% — directionally wrong; requirements increase rather than fall in pregnancy (anchoring)

Additional high-yield points

  • Check TSH approximately every 4 weeks through the first half of pregnancy and titrate to trimester-specific targets
  • Return to the prepregnancy dose postpartum
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Side-by-side comparison

FeatureHyperthyroidismHypothyroidism
At a glanceAutoimmune diffuse goiter driven by TSH-receptor stimulating antibodies; most common cause of overt hyperthyroidism.Autoimmune chronic lymphocytic thyroiditis; most common cause of hypothyroidism in iodine-replete regions.
Classic presentationYoung woman with weight loss, palpitations, anxiety, diffuse goiter with bruit, and proptosis = Graves disease.; Heat intolerance, sweating, palpitations, tremor, anxiety, insomnia; Unintentional weight loss despite increased appetite; Frequent loose stools, oligomenorrhea, proximal muscle weakness; Gritty, bulging eyes; diplopia;…Middle-aged woman with fatigue, weight gain, cold intolerance, diffuse firm bumpy goiter, and delayed reflex relaxation.; Fatigue, cold intolerance, weight gain, constipation; Dry skin, hair loss (lateral third of eyebrows), brittle nails; Menstrual irregularity (menorrhagia in early disease), infertility; Depression, cognitive slowing,…
Workup / key labsSuppressed TSH + elevated free T4 and/or T3 + positive TRAb, or diffuse uptake on RAIU scan. Orbitopathy or pretibial myxedema with biochemical thyrotoxicosis is sufficient even without antibody confirmation.; TSH (suppressed, <0.01 in overt disease) — most sensitive initial test; Free T4 and total T3 (elevated; T3-predominant toxicosis…Elevated TSH with low free T4 (overt) or normal free T4 (subclinical), supported by positive anti-TPO antibodies.; TSH — elevated (most sensitive screening test); Free T4 — low in overt disease; normal in subclinical disease; Anti-TPO antibodies — positive in 90-95% of Hashimoto cases (also anti-thyroglobulin in ~60%); Lipid panel…
ImagingRadioactive iodine uptake and scan (RAIU/scan) — diffuse, homogeneous, ELEVATED uptake confirms Graves; differentiates from low-uptake thyroiditis and factitious; Thyroid ultrasound with color Doppler — hypervascular (thyroid inferno) in Graves; useful when RAIU contraindicated (pregnancy, lactation, recent iodine); Orbital CT/MRI…Ultrasound only if nodule, asymmetry, or compressive symptoms — heterogeneous hypoechoic pattern with pseudonodules typical of Hashimoto; RAIU not routinely indicated
First-line treatmentBeta-blocker for adrenergic symptoms — propranolol 20-40 mg q6h (also blocks peripheral T4→T3 conversion at high dose) or atenolol/metoprolol; use cautiously in HF; Thionamide — methimazole 10-40 mg daily (first-line in non-pregnancy; longer half-life; less hepatotoxicity); propylthiouracil (PTU) preferred in 1st trimester pregnancy and…Levothyroxine (synthetic T4) — start 1.6 mcg/kg/day in healthy young adults; start lower in the elderly (25-50 mcg) and lowest in known CAD (12.5-25 mcg); take on empty stomach 30-60 min before food or 4 h after; separate from calcium, iron, PPIs, fiber, soy; Recheck TSH at 6-8 weeks after initiation or dose change; goal TSH within the…

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Full outlines

Hyperthyroidism (Graves Disease) Hypothyroidism (Hashimoto Thyroiditis) All 514 diagnoses

Educational use only. This outline is a study aid for PA students and is not medical advice or a substitute for clinical judgment. FirstPassPA is an independent study tool and is not affiliated with, endorsed by, or sponsored by NCCPA or PAEA. PANCE® and PANRE® are registered trademarks of the National Commission on Certification of Physician Assistants; End of Rotation™ is a program of the Physician Assistant Education Association.