High-Yield Endocrinology for USMLE Step 3: Diabetes, Thyroid, and Adrenal Emergencies

I once watched a senior resident stop an insulin drip because the glucose hit 180. The patient bounced back into DKA twelve hours later. That single teaching moment — the gap between "glucose is fixed" and "the metabolic derangement is fixed" — captures what makes endocrinology on Step 3 deceptively hard. The questions aren't asking you to memorize lab values. They're asking whether you actually understand the physiology well enough to not kill someone.

Let's walk through what matters.

Diabetes Mellitus

Type 2 Diabetes: Drug Selection Hierarchy

Step 3 doesn't just test whether you know metformin is first-line. It tests whether you can pick the right second agent when a patient walks in with heart failure, a creatinine of 2.3, and an A1c of 9.

First-line (always): Metformin — unless eGFR is under 30, the liver is failing, or contrast is on the schedule.

The add-on decision is comorbidity-driven, not glucose-driven:

• Established ASCVD or high CV risk: GLP-1 receptor agonist (liraglutide, semaglutide) OR SGLT-2 inhibitor (empagliflozin, dapagliflozin). Both have hard cardiovascular mortality data behind them.
• Heart failure or CKD: SGLT-2 inhibitor. Full stop. Reduces HF hospitalizations and slows CKD progression in ways that genuinely surprised the cardiology world.
• Obesity: GLP-1 receptor agonist — pulls double duty on weight and glucose.
• Cost is the limiting factor: sulfonylurea (glipizide, glimepiride) or NPH insulin. Not glamorous, but effective.

What catches people off guard: SGLT-2 inhibitors now carry an indication in heart failure regardless of ejection fraction — both HFrEF and HFpEF — and regardless of whether the patient even has diabetes. This crossed specialty lines in a way that Step 3 loves to exploit.

Glycemic targets — and why they aren't one-size-fits-all:

• HbA1c < 7% for most patients
• < 8% for elderly patients, those with limited life expectancy, or anyone with recurrent hypoglycemia
• Tighter control (< 6.5%) only for young, otherwise healthy patients early in their disease course

Here's a contrarian take I stand behind: we over-treat A1c in the elderly. I've seen attendings chase a 7.0 target in an 82-year-old with dementia and recurrent falls, stacking insulin on top of a sulfonylurea. The resulting hypoglycemia is more dangerous than the hyperglycemia. Step 3 increasingly tests this — the answer is sometimes "relax the target," not "add another drug."

DKA vs. HHS: Know the Pattern, Not Just the Numbers

Feature	DKA	HHS
Patient type	Type 1 DM (typically)	Type 2 DM (typically)
Glucose	Usually 250–800 mg/dL	> 600 mg/dL
Anion gap	Elevated (metabolic acidosis)	Normal or mild
Ketones	Large	Absent or trace
Osmolality	Normal or mildly elevated	> 320 mOsm/kg
Mental status	Mild changes	Profound obtundation

The distinction that matters clinically: DKA is an acid problem. HHS is an osmolality problem. A patient can have a glucose of 400 in DKA and be more metabolically deranged than someone at 900 in HHS. The anion gap tells you severity in DKA — the glucose doesn't.

DKA Management: The Sequence That Saves Lives

1. Volume first. NS at 1–2L over the first hour, then transition to 0.45% NS at 250–500 mL/hr. These patients are profoundly dry — often 5 to 8 liters behind.
2. Insulin drip at 0.1 units/kg/hr — but check potassium first. If K+ is below 3.5, you replete potassium before starting insulin. Ignoring this step kills people.
3. Potassium goes into the fluids once K+ drops below 5.3 and urine output is confirmed. Insulin shoves potassium intracellularly; the serum level will plummet.
4. When glucose drops below 250, add dextrose to the IV fluids. Do NOT stop the insulin. The drip continues until the anion gap closes and the patient is eating.
5. Bicarbonate only if pH falls below 6.9. This threshold is more conservative than people expect.
6. Find the trigger. Infection tops the list. Then missed insulin doses, new-onset DM, substance use, pancreatitis.

The single most tested DKA pearl: Stopping the insulin drip when glucose normalizes is wrong. Glucose corrects hours before the acidosis resolves. You keep the drip running, feed it dextrose, and wait for the gap to close. I cannot overstate how often this shows up.

Thyroid Disease

Hypothyroidism

Primary hypothyroidism — Hashimoto thyroiditis is the most common etiology in the US.

• TSH elevated, free T4 low. Straightforward.
• Levothyroxine is the treatment. Start low and go slow in elderly patients and anyone with coronary disease — you don't want to unmask angina.
• Recheck TSH every 6–8 weeks. Adjust by 12.5–25 mcg increments. Resist the urge to recheck at 3 weeks; the TSH hasn't equilibrated yet.

Secondary hypothyroidism from pituitary failure flips the script:

• TSH is low (or inappropriately normal), free T4 is low.
• Here's the trap: you cannot use TSH to monitor treatment. The pituitary is broken. Follow free T4 instead.

Myxedema coma is the endocrine emergency people forget to look for. Altered mental status, hypothermia, bradycardia, hypoventilation — often precipitated by infection, cold exposure, or sedative medications in an undertreated hypothyroid patient.

• IV levothyroxine (or T3 for faster onset) plus IV hydrocortisone. You cover for concurrent adrenal insufficiency because pituitary pathology can knock out both axes simultaneously.
• ICU admission. Passive rewarming. Active warming is dangerous here — peripheral vasodilation can crash an already tenuous blood pressure.

Hyperthyroidism

Graves disease dominates this category: TSH receptor-stimulating antibodies produce a classic triad of diffuse goiter, ophthalmopathy, and pretibial myxedema.

Diagnosis: TSH suppressed, free T4 and/or T3 elevated, positive TRAb or TSI.

Treatment — and knowing which drug goes where:

• Methimazole is preferred in almost every scenario.
• PTU gets the nod in two specific situations: first trimester of pregnancy (methimazole is teratogenic during organogenesis) and thyroid storm (PTU blocks peripheral T4→T3 conversion, which methimazole doesn't).
• Propranolol controls symptoms fast — tremor, tachycardia, anxiety — but does nothing to the underlying thyroid hormone production. Think of it as buying time.
• Radioactive iodine: definitive treatment, avoids surgery, but absolutely contraindicated in pregnancy and problematic with active Graves ophthalmopathy (can worsen it).
• Thyroidectomy for compressive goiters, medication failures, or when RAI isn't an option.

Thyroid storm management has a specific order, and the order matters:

1. PTU first (or methimazole) — blocks new hormone synthesis
2. Potassium iodide or Lugol's solution — given one hour after the thionamide. If you give iodine first, you feed the fire before blocking the furnace.
3. High-dose propranolol (IV or oral) — rate control and blocks peripheral T4→T3 conversion
4. Hydrocortisone — inhibits T4→T3 conversion and covers for relative adrenal insufficiency in a catabolic crisis
5. Cooling measures and supportive ICU care

Surprising detail worth knowing: The one-hour delay between PTU and iodine administration isn't arbitrary protocol — if you give iodine to an unblocked thyroid gland, it uses that iodine to synthesize even more hormone (Jod-Basedow phenomenon). This sequencing detail has shown up on multiple exam forms.

Adrenal Disease

Adrenal Insufficiency and Adrenal Crisis

Primary adrenal insufficiency (Addison disease):

• Autoimmune destruction of the adrenal cortex is the leading cause in developed countries. TB and metastatic infiltration still matter globally.
• You lose everything the adrenal cortex makes: cortisol, aldosterone, and androgens.
• Labs paint a clear picture: low cortisol, sky-high ACTH (the pituitary is screaming at a gland that can't respond), hyperkalemia, hyponatremia, eosinophilia.
• Hyperpigmentation is the physical exam finding that clinches it — MSH is co-secreted with ACTH from the same precursor molecule (POMC). The darker the skin creases, the higher the ACTH.

Secondary adrenal insufficiency:

• Pituitary fails → ACTH drops → cortisol drops. But aldosterone is preserved because the renin-angiotensin system controls it independently.
• The most common real-world cause: a patient who was on chronic prednisone gets tapered too quickly. Their HPA axis is suppressed and can't mount a cortisol response. This is far more common than Addison disease, and Step 3 knows it.

Adrenal crisis hits when a physiologically stressed patient — surgery, sepsis, trauma — can't mount an appropriate cortisol surge:

• Refractory hypotension that doesn't respond to fluids or pressors. Hypoglycemia. Nausea, confusion, circulatory collapse.
• Do not wait for a cortisol level in an unstable patient. Draw the blood, then treat immediately.
• IV hydrocortisone 100 mg bolus, followed by 50–100 mg every 6–8 hours, plus aggressive NS resuscitation. At stress doses, hydrocortisone has enough mineralocorticoid activity that you don't need separate fludrocortisone acutely.

Cushing Syndrome

Most common cause overall: exogenous corticosteroid use. Always ask about steroids — including inhaled, topical, and intra-articular injections — before launching a workup.

Most common endogenous cause: pituitary adenoma (Cushing disease, ACTH-dependent).

Clinical features cluster around cortisol excess: central obesity, moon facies, buffalo hump, violaceous striae, proximal muscle weakness, hypertension, hyperglycemia, osteoporosis, thin skin with easy bruising. The striae being violaceous (not white) and proximal weakness are the features that most reliably distinguish Cushing from simple obesity.

The diagnostic algorithm is three steps — don't skip ahead:

1. Confirm hypercortisolism: 24-hour urine free cortisol, 1 mg overnight dexamethasone suppression test, or late-night salivary cortisol. Any one can screen; two concordant positives make the diagnosis strong.
2. Check ACTH: Elevated ACTH = ACTH-dependent (pituitary adenoma vs. ectopic source like small cell lung cancer). Suppressed ACTH = ACTH-independent (adrenal adenoma or adrenal carcinoma).
3. Localize the ACTH-dependent source: High-dose dexamethasone suppression test plus pituitary MRI. Pituitary adenomas typically suppress with high-dose dex; ectopic sources don't. Inferior petrosal sinus sampling is the gold standard when imaging is equivocal.

Frequently Asked Questions

Why do SGLT-2 inhibitors work in heart failure even without diabetes?

The mechanism isn't fully settled, but the leading theory involves osmotic diuresis, reduced preload, and direct effects on cardiac sodium-hydrogen exchange. What's clear from the EMPEROR-Preserved and DELIVER trials is that the benefit is real and independent of glucose lowering. For Step 3, know the indication — you don't need to explain the mechanism.

How do I distinguish subclinical hypothyroidism from a lab artifact on the exam?

Subclinical hypothyroidism shows an elevated TSH with a normal free T4. The key question is whether to treat. Current guidelines favor treatment when TSH exceeds 10, or when TSH is between 5 and 10 with symptoms or pregnancy planning. A mildly elevated TSH in an elderly patient may be physiologically normal and doesn't warrant levothyroxine.

Can a patient present in DKA with a normal or near-normal glucose?

Yes — this is euglycemic DKA, and SGLT-2 inhibitors made it a tested topic. These drugs cause urinary glucose wasting, so patients can develop significant ketoacidosis with glucose levels under 250. If a patient on an SGLT-2 inhibitor presents with anion gap metabolic acidosis, check ketones regardless of the glucose.

What's the most commonly missed trigger for adrenal crisis on Step 3?

Abrupt discontinuation of chronic corticosteroids — especially in surgical or acute illness settings. The classic stem describes a patient on long-term prednisone who undergoes surgery and becomes hypotensive postoperatively despite adequate fluids. Stress-dose steroids are the answer.

When should I suspect ectopic ACTH rather than a pituitary source in Cushing syndrome?

Rapid onset of severe symptoms, profound hypokalemia, and very high ACTH levels (often >200 pg/mL) should raise suspicion for an ectopic source. These patients often look more sick than cushingoid. Small cell lung cancer is the classic association, but carcinoid tumors can produce a more indolent picture that mimics pituitary disease.

Practice Endocrinology Questions

Ready to put this to the test? Step3Sim offers free USMLE Step 3 practice questions covering endocrinology and every other organ system — built around the same management-focused reasoning the real exam demands.