Endocrinology
Hormonal Feedback
Negative feedback loops that keep hormone levels in range
The endocrine system maintains homeostasis through negative feedback loops. The hypothalamus releases releasing hormones (TRH, CRH, GnRH, GHRH); the anterior pituitary releases tropic hormones (TSH, ACTH, LH/FSH, GH); peripheral glands release end-organ hormones (T3/T4, cortisol, sex steroids, IGF-1) that feedback to suppress the upstream signals. Examples — high T4 suppresses TSH and TRH; high cortisol suppresses ACTH and CRH. Positive feedback is rare but occurs at midcycle (estrogen surge triggers LH surge for ovulation) and during labor (oxytocin and uterine stretch). Disease arises from primary failure (gland), secondary failure (pituitary), or tertiary failure (hypothalamic). Knowing the level localizes the lesion.
- Master regulatorHypothalamus
- Common loopHypothalamus → pituitary → gland → feedback
- Time scaleMinutes (insulin) to hours (TSH) to weeks (T4 half-life ~7 days)
- Primary failureLow end-hormone, high tropic (e.g., low T4, high TSH)
- Secondary failureLow end-hormone and low tropic (pituitary)
- Tertiary failureHypothalamic; low releasing hormone
Interactive visualization
Press play, or step through manually. The visualization is yours to drive — try it before reading on.
Watch the 60-second explainer
A condensed visual walkthrough — narrated, captioned, under a minute.
Why feedback matters
- Localizing endocrine disease. Tropic vs end-organ levels pinpoint the failure level.
- Steroid withdrawal. Adrenal suppression risk after >2 weeks supraphysiologic dose.
- Thyroid dosing. Adjust levothyroxine to TSH target.
- Cushing workup. Sequential testing maps the lesion.
- Fertility evaluation. FSH and LH localize ovulation problem.
- Diabetes types. C-peptide distinguishes type 1 vs type 2.
- Pituitary disease. Multi-axis testing for panhypopituitarism.
Common errors
- Treating sick euthyroid with levothyroxine. Worsens recovery; resolve underlying illness.
- Stopping chronic steroids abruptly. Adrenal crisis risk; taper after >2 weeks therapy.
- Misreading TSH in central hypothyroidism. Normal TSH with low T4 is pituitary disease.
- Ignoring biotin interference. Skews TSH, T4, troponin, hCG; hold supplement 48 hours.
- Trusting morning cortisol alone. Stress, illness elevate; suppression test if Cushing suspected.
- Forgetting to taper after pulse steroids. Adrenal axis recovery takes weeks to months.
Frequently asked questions
How does the HPA axis work?
Hypothalamus releases corticotropin-releasing hormone (CRH) in response to stress, hypoglycemia, illness, circadian rhythm. CRH stimulates anterior pituitary to release ACTH. ACTH stimulates adrenal cortex to release cortisol. Cortisol provides negative feedback to hypothalamus and pituitary. Diurnal pattern — peak 6-8 AM, trough at midnight. Stress and illness override normal feedback. Suppression by exogenous steroids causes adrenal atrophy; abrupt withdrawal triggers crisis.
How is hypothyroidism diagnosed?
Primary hypothyroidism — TSH high, free T4 low (most common, e.g., Hashimoto thyroiditis); TPO antibodies typical. Subclinical — TSH high, free T4 normal; treat if TSH >10 or symptoms or pregnancy plans. Secondary (central) — TSH low or inappropriately normal, free T4 low; pituitary or hypothalamic disease. Treatment — levothyroxine 1.6 µg/kg/day, titrate to TSH 0.5-2.5 (or normal range; target free T4 in central hypothyroidism since TSH unreliable).
What is the dexamethasone suppression test?
Tests negative feedback in HPA axis. 1 mg dexamethasone at 11 PM; cortisol at 8 AM — should suppress to <1.8 µg/dL. Failure suggests Cushing syndrome. Causes — pituitary adenoma (Cushing disease, partial suppression at high dose), ectopic ACTH (small cell lung, no suppression), adrenal tumor (low ACTH, no suppression). Refine with ACTH level, high-dose dex, CRH stimulation, MRI, IPSS. Pseudo-Cushing — depression, alcoholism, obesity — confound.
How does feedback fail in disease?
Primary disease — gland fails; tropic hormone rises trying to compensate. Hashimoto thyroiditis — autoimmune destruction; T4 low, TSH high. Addison disease — adrenal destruction; cortisol low, ACTH high (hyperpigmentation from MSH-like activity). Secondary — pituitary disease; both tropic and end-organ low. Sheehan syndrome (postpartum pituitary necrosis), tumor, surgery, radiation. Tertiary — hypothalamic; releasing hormone deficient.
What is positive feedback?
Output amplifies its own production. LH surge — high estradiol from mature follicle switches feedback from negative to positive at hypothalamus and pituitary, triggering LH surge that causes ovulation. Oxytocin — uterine stretch during labor releases oxytocin, intensifying contractions, more stretch, more oxytocin until delivery. Breastfeeding letdown reflex similar. Most feedback is negative; positive loops are self-limiting and tied to discrete events.
What are paradoxical lab patterns?
Sick euthyroid (nonthyroidal illness) — low T3, low/normal T4, TSH low or normal. Don't treat with levothyroxine. Resolves with recovery. Hashitoxicosis — early Hashimoto with thyroid hormone leak; transient hyperthyroidism then hypothyroidism. Subacute thyroiditis — pain, viral prodrome, transient hyper then hypo. Postpartum thyroiditis. Lithium and amiodarone induce thyroid dysfunction. Always consider biotin interference with hormone immunoassays — discontinue 48 hours before testing.
How does insulin feedback work?
Glucose stimulates beta-cell insulin release; insulin promotes uptake and storage, lowering glucose; falling glucose reduces insulin; counterregulation by glucagon, cortisol, GH, epinephrine raises glucose if too low. Type 1 — autoimmune beta-cell destruction; absolute insulin deficiency. Type 2 — insulin resistance and progressive beta-cell failure. Insulin levels paradoxically low or normal despite hyperglycemia in type 2 (relative deficiency). C-peptide measures endogenous insulin production.