Step 1 topics → Endocrine → Hypothyroidism (Primary, Cretinism, Drug-Induced)

Hypothyroidism (Primary, Cretinism, Drug-Induced)

USMLE Step 1 trap: Interprets low TSH as hyperthyroidism without considering central hypothyroidism when free T4 is also low. Low TSH with low free T4 indicates central (secondary or tertiary) hypothyroidism, not hyperthyroidism.

Hypothyroidism is one of the highest-yield endocrine topics on USMLE Step 1, and students consistently misinterpret a low TSH as evidence of hyperthyroidism — forgetting that central hypothyroidism (pituitary or hypothalamic failure) produces low TSH alongside low free T4. The exam tests the full differential of causes (Hashimoto's, iodine deficiency, lithium, amiodarone, post-thyroidectomy), the classic multisystem presentation, congenital hypothyroidism (cretinism), and levothyroxine monitoring — including the 6-8 week recheck interval that students routinely ignore.

The trickiest part is lab interpretation. Students default to 'low TSH = hyperthyroid, high TSH = hypothyroid,' but that heuristic breaks down in central hypothyroidism (pituitary or hypothalamic failure), where TSH is low or inappropriately normal and free T4 is also low. USMLE Step 1 absolutely exploits this. You also need to understand that TSH is the best screening test for primary hypothyroidism but not for central causes — free T4 is what tells you the true thyroid status in ambiguous cases.

Cretinism gets its own angle because the exam wants you to know that iodine deficiency — not genetic mutation — is the leading global cause, and that newborn screening exists precisely because the baby looks normal at birth (maternal T4 crosses the placenta and compensates during fetal development). Amiodarone and lithium as drug causes are classic Step 1 traps. Hashimoto's antibodies (anti-TPO, anti-thyroglobulin) and its histology (lymphocytic infiltration, Hürthle cell change, germinal centers) round out the high-yield picture.

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Common misconceptions

Common mistake
Wrong: Low TSH always indicates hyperthyroidism.
Right: Low TSH with low free T4 indicates central (secondary or tertiary) hypothyroidism, not hyperthyroidism.
Low TSH does not automatically mean hyperthyroidism — you must always check free T4 alongside TSH. In central (secondary or tertiary) hypothyroidism, the pituitary fails to produce adequate TSH, so TSH is low or inappropriately normal even though the patient is hypothyroid and free T4 is low. The key move on the exam is: if TSH is low and free T4 is also low, think pituitary or hypothalamic pathology, not hyperthyroidism.
Common mistake
Wrong: Cretinism is caused solely by genetic defects in thyroid hormone synthesis.
Right: The most common worldwide cause of cretinism is maternal iodine deficiency during pregnancy, not a genetic defect.
Cretinism is not primarily a genetic disease — in global terms, maternal iodine deficiency during pregnancy is the dominant cause, which is why iodine supplementation programs exist. The fetus depends entirely on maternal iodine to synthesize thyroid hormone for brain development, so severe iodine deficiency causes irreversible neurologic damage. Genetic defects in thyroid hormone synthesis (e.g., dyshormonogenesis) are a real but far less common cause, and confusing the two will cost you on a population-health or etiology question.
Common mistake
Wrong: TSH should be rechecked within days of starting levothyroxine to assess adequacy.
Right: TSH should be rechecked 6–8 weeks after initiating or adjusting levothyroxine because the pituitary TSH response lags behind circulating T4 normalization.
TSH response lags significantly behind changes in circulating T4 because the pituitary thyrotrophs take time to recalibrate after prolonged suppression or elevation. Rechecking TSH within days of starting or adjusting levothyroxine gives you a meaningless number — the pituitary has not had time to respond. The standard monitoring interval is 6–8 weeks, and this is a specific fact USMLE Step 1 tests in management questions.
Common mistake
Gap: Misses that levothyroxine requirements increase significantly during pregnancy with a tighter TSH target
Pregnant women with hypothyroidism require higher levothyroxine doses (often 25–50% increase) and a lower TSH target to support fetal neurodevelopment.
Pregnancy dramatically increases levothyroxine requirements — often by 25–50% — because rising estrogen increases thyroid-binding globulin, the placenta degrades T4, and fetal thyroid development demands additional substrate. The TSH target in pregnancy is lower than in non-pregnant adults (typically <2.5 mIU/L in the first trimester) because even subclinical hypothyroidism impairs fetal neurodevelopment. Any vignette involving a hypothyroid woman who becomes pregnant should trigger immediate dose reassessment.
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What the exam tests

  1. Distinguish the major etiologies of hypothyroidism — including Hashimoto's thyroiditis, iodine deficiency, lithium, amiodarone, post-surgical, and central causes — and recognize which lab pattern (TSH/free T4 combination) corresponds to each.
  2. Identify the classic multisystem symptoms of adult hypothyroidism: fatigue, cold intolerance, weight gain, constipation, bradycardia, dry skin, coarse hair, myxedema, delayed deep tendon reflex relaxation, and hyponatremia.
  3. Recognize congenital hypothyroidism (cretinism) — its features (intellectual disability, short stature, macroglossia, umbilical hernia, prolonged neonatal jaundice), why newborn screening is performed before symptoms appear, and that iodine deficiency is the leading worldwide cause.
  4. Apply levothyroxine dosing principles: TSH is the monitoring target, recheck interval is 6–8 weeks (not days) after initiation or dose change, and pregnant patients require a 25–50% dose increase with a tighter TSH target to protect fetal neurodevelopment.

Can you avoid these mistakes?

A 34-year-old woman is found to have TSH of 0.2 mIU/L (low) and free T4 of 0.4 ng/dL (low). She has fatigue and weight gain. What is the most likely diagnosis, and what anatomical level of the HPT axis is affected?
A newborn in a developing country is born to a mother with severe nutritional deficiency. At 6 months, the infant has intellectual disability, macroglossia, and short stature. What is the most common global cause of this presentation, and why did the newborn appear normal at birth?
You start a 45-year-old man on levothyroxine for newly diagnosed primary hypothyroidism. When should you recheck his TSH, and why is rechecking at 2 weeks inadequate?
A woman with known Hashimoto's hypothyroidism on a stable levothyroxine dose reports her TSH has risen to 4.8 mIU/L at her 10-week prenatal visit. Her pre-pregnancy TSH was 1.9 mIU/L. What should you do, and what is the rationale?

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Hypothalamic-Pituitary-Thyroid (HPT) Axis Hypothalamic-Pituitary-Adrenal (HPA) Axis Hypothalamic-Pituitary-Gonadal (HPG) Axis Hormone Signaling (Peptide vs Steroid)

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