Physiology · Year 1 · from Physiology

Case 2: Primary Hypothyroidism - Interpreting Hormone Levels with Feedback

Clinical Image

Source: Wikimedia Commons - Goiter - CC BY-SA 3.0

Patient Presentation

A 35-year-old woman presents with a 4-month history of fatigue, weight gain of 15 pounds, constipation, and feeling cold all the time. She also reports dry skin, hair loss, and difficulty concentrating at work. Her menstrual periods have become heavier and more frequent.

Demographics

  • Age: 35 years
  • Sex: Female
  • Past Medical History: None significant
  • Family History: Mother with "thyroid problems"

Chief Complaint

Fatigue, weight gain, cold intolerance, and constipation

Physical Examination

  • Blood pressure: 108/72 mmHg
  • Heart rate: 56 bpm (bradycardic)
  • Temperature: 36.2°C (low normal)
  • General: Appears fatigued, mild periorbital puffiness
  • Thyroid: Diffusely enlarged, firm, non-tender
  • Skin: Cool, dry skin; brittle nails
  • Reflexes: Delayed relaxation phase (hung-up reflexes)

Workup

  • TSH: 48 mIU/L (markedly elevated; normal 0.4-4.0 mIU/L)
  • Free T4: 0.4 ng/dL (low; normal 0.8-1.8 ng/dL)
  • Anti-TPO antibodies: Positive (elevated titer)
  • Anti-thyroglobulin antibodies: Positive
  • Lipid panel: Total cholesterol 268 mg/dL, LDL 172 mg/dL (elevated)

Diagnosis

Primary Hypothyroidism due to Hashimoto's Thyroiditis

Treatment

  1. Levothyroxine replacement therapy (starting dose 1.6 mcg/kg/day)
  2. Recheck TSH in 6-8 weeks and titrate dose
  3. Goal: Normalize TSH
  4. Education about lifelong therapy and medication interactions
  5. Screen for other autoimmune conditions

Physiological Principles Demonstrated

  • Primary vs. secondary endocrine disorders: The pattern of low T4 with elevated TSH indicates the problem is at the thyroid gland (primary), not the pituitary. Intact negative feedback causes TSH to rise in response to low thyroid hormone.
  • Negative feedback in action: The elevated TSH demonstrates the pituitary's appropriate response to low circulating thyroid hormone - the feedback loop is intact but the thyroid cannot respond.
  • Thyroid hormone effects: The clinical features reflect decreased metabolic rate throughout the body: bradycardia, cold intolerance, constipation, weight gain, and mental slowing.
  • Autoimmune pathophysiology: TPO antibodies indicate autoimmune destruction of the thyroid gland, a common cause of primary hypothyroidism.

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