Endocrinology

Thyroid Disorders

Clinical overview and exam mastery guide for hypothyroidism, hyperthyroidism, thyroid storm sequence, and levothyroxine/antithyroid drug strategy.

Most Sensitive Test
TSH
Hypothyroid Core Drug
Levothyroxine
Hyperthyroid First-Line
Methimazole
Storm Sequence
PTU then iodine

TSH Axis and Drug Targets

TRH Hypothalamus regulatory start TSH Pituitary output sensitive marker T4/T3 Thyroid hormone negative feedback Treatment pivots on under- vs over-production Replace hormone in hypothyroidism, suppress synthesis/release in hyperthyroidism

Drug sequencing is especially critical in thyroid storm.

1. Thyroid Physiology (Foundation First)

Thyroid gland produces T4 and T3 (active form). Regulation follows hypothalamic TRH to pituitary TSH to thyroid hormone release, with T3/T4 negative feedback on TSH.

High-yield diagnostic rule: TSH is the most sensitive screening test.

2. Hypothyroidism

Common Causes

  • Hashimoto thyroiditis (most common)
  • Post-thyroidectomy or radioiodine
  • Iodine deficiency
  • Drug-related (e.g., amiodarone)

Clinical Features

  • Weight gain, fatigue, cold intolerance
  • Constipation, bradycardia, dry skin
  • Severe emergency: myxedema coma

3. Management of Hypothyroidism

Levothyroxine (T4)

  • MOA: synthetic T4 replacement converted peripherally to T3
  • Dosing: take on empty stomach; separate calcium/iron by at least 4 hours
  • Monitoring: recheck TSH every 6 to 8 weeks after dose changes
  • Over-replacement effects: tachycardia, arrhythmias, long-term bone loss risk
  • Contra/caution: untreated adrenal insufficiency; cautious titration in elderly/CAD

4. Myxedema Coma (Emergency)

Typical Features

  • Hypothermia
  • Bradycardia
  • Altered mental status

Treatment Core

  • IV levothyroxine
  • IV hydrocortisone until adrenal insufficiency is excluded
  • Supportive ICU-level care

5. Hyperthyroidism

Causes

  • Graves disease (most common)
  • Toxic multinodular goiter
  • Thyroiditis
  • Amiodarone-associated thyroid dysfunction

Clinical Features

  • Weight loss, heat intolerance, tremor
  • Palpitations, anxiety, diarrhea

6. Management of Hyperthyroidism

Core options: antithyroid drugs, radioactive iodine, or surgery.

Drug/Class MOA Major Side Effects Contraindications / Notes
Methimazole (first-line) Inhibits thyroid peroxidase and blocks T3/T4 synthesis Rash, agranulocytosis, hepatotoxicity Avoid in first-trimester pregnancy (use PTU)
Propylthiouracil (PTU) Inhibits thyroid hormone synthesis and peripheral T4 to T3 conversion Hepatotoxicity (greater severity risk), agranulocytosis Preferred in first trimester and thyroid storm contexts
Beta blockers (e.g., propranolol) Beta blockade reduces adrenergic symptoms; propranolol also lowers T4 to T3 conversion Bradycardia, fatigue, hypotension Symptom control in most hyperthyroid presentations

7. Thyroid Storm (Life-Threatening)

Features

  • High fever
  • Tachycardia
  • Delirium/encephalopathy
  • Heart failure risk

Critical Exam Sequence

  • 1. PTU
  • 2. Iodine (after PTU)
  • 3. Beta blocker
  • 4. Steroid
Iodine must be given after PTU in storm management to avoid fueling new hormone synthesis.

8. Subclinical Thyroid Disease

Defined by abnormal TSH with normal circulating T3/T4. Treatment decisions depend on TSH magnitude, symptoms, age, and comorbidity profile.

Management Recap Drill

Hypothyroidism: start levothyroxine and monitor TSH.
Hyperthyroidism: methimazole first-line (except first trimester).
PTU role: first trimester and thyroid storm.
Storm sequence: PTU then iodine, plus beta blocker and steroid.

Visual Algorithm Placeholder

[Insert Thyroid Disorder Diagnosis and Treatment Algorithm Here During UI Integration]

Guideline References (Management)

American Thyroid Association Guidelines

https://www.thyroid.org

Guideline Scope

  • Levothyroxine dosing and monitoring
  • Hyperthyroidism treatment pathways
  • Thyroid storm protocol

9. Common Exam Traps

Methimazole is first-line except in first trimester.
PTU inhibits peripheral T4 to T3 conversion.
Iodine must follow PTU in thyroid storm.
TSH is usually the most sensitive initial test.
Levothyroxine absorption is reduced by food, calcium, and iron.

10. Quick Revision Summary

Must Remember

  • TSH guides screening and dose adjustment
  • Hypothyroidism treatment is levothyroxine replacement
  • Hyperthyroidism usually starts with methimazole
  • Thyroid storm starts with PTU before iodine
  • Beta blockers control adrenergic symptom burden

Practice Questions Placeholder

  • Topic: Thyroid Disorders
  • Subtopics: hypothyroidism, hyperthyroidism, methimazole, PTU, thyroid storm, levothyroxine monitoring