# Clinical Cases: Thyroid Anatomy and Physiology

## Case 1: Thyroglossal Duct Cyst

### Patient Demographics
- **Age:** 8 years
- **Sex:** Male
- **Occupation:** Student (3rd grade)

### Chief Complaint
"There's a bump in the middle of my son's neck that moves when he swallows."

### History of Present Illness
An 8-year-old boy is brought by his mother who noticed a painless midline neck mass about 3 weeks ago. The mass has been slowly enlarging. The child denies pain, difficulty swallowing, or voice changes. He has had no fever or recent upper respiratory infections. The mass appears to move upward when he swallows or sticks out his tongue. There is no family history of thyroid disease.

### Physical Examination
- **Vital Signs:** Normal for age
- **General:** Well-appearing boy in no distress
- **Neck:**
  - 2 cm smooth, round, non-tender mass in anterior midline
  - Located between the hyoid bone and thyroid cartilage
  - Mass elevates with swallowing and tongue protrusion (positive Sistrunk sign)
  - No overlying skin changes
  - Thyroid gland palpable in normal position
- **Lymph nodes:** No cervical lymphadenopathy

### Workup
- **Laboratory Studies:**
  - TSH: 2.1 mIU/L (normal)
  - Free T4: 1.2 ng/dL (normal)
- **Ultrasound Neck:**
  - 2.2 cm anechoic (cystic) midline mass
  - Located inferior to hyoid bone
  - Normal thyroid gland in expected location
- **Thyroid Scan (if concern for ectopic thyroid):** Not needed in this case as normal thyroid identified on ultrasound

### Diagnosis
**Thyroglossal duct cyst**

### Treatment
1. **Sistrunk procedure:**
   - Surgical excision of the cyst
   - Includes excision of the central portion of the hyoid bone
   - Removal of tissue along the tract up to the foramen cecum (base of tongue)
2. **Rationale:** Complete tract removal reduces recurrence rate from 50% to <5%
3. **Post-operative:** Monitor for wound complications, hypothyroidism (rare)
4. **If infected at presentation:** Antibiotics first, then elective surgery after resolution

### Clinical Pearl
The thyroglossal duct is the embryologic tract through which the thyroid gland descends from the foramen cecum at the base of the tongue to its final position in the anterior neck. Failure of this tract to obliterate leads to thyroglossal duct cyst, the most common congenital neck mass in children. The pathognomonic finding is elevation of the mass with tongue protrusion, which occurs because the tract connects to the base of the tongue. Before surgical removal, it is essential to confirm that a normal thyroid gland exists in the proper location, as the cyst may contain the only functioning thyroid tissue in rare cases (1-2%).

### Clinical Image
![Thyroglossal Duct Cyst](case_01_image.jpg)

*Clinical photograph showing a midline neck mass in the typical location of a thyroglossal duct cyst.*

**Image Source:** Wikimedia Commons - "Thyroglossal cyst"
**License:** CC BY-SA 4.0
**URL:** https://commons.wikimedia.org/wiki/File:Thyroglossal_cyst.jpg

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## Case 2: Lingual Thyroid

### Patient Demographics
- **Age:** 16 years
- **Sex:** Female
- **Occupation:** High school student

### Chief Complaint
"I feel like there's something in the back of my throat, and I sometimes have trouble swallowing."

### History of Present Illness
A 16-year-old girl presents with a 6-month history of sensation of a mass in her throat, mild dysphagia with solid foods, and occasional gagging. She has not had voice changes, pain, or bleeding. She reports that she has always been somewhat tired and has had difficulty with weight despite diet and exercise. She started menstruating at age 14 but her periods have been irregular and infrequent. On review of systems, she notes constipation, dry skin, and cold intolerance.

### Physical Examination
- **Vital Signs:** BP 108/70 mmHg, HR 58 bpm
- **General:** Mildly overweight, appears fatigued
- **HEENT:**
  - Oral cavity: Smooth, rounded, pink-red mass at base of tongue (posterior midline)
  - Mass approximately 2 cm, non-tender, covered with normal mucosa
- **Neck:** Thyroid gland NOT palpable in normal pre-tracheal position
- **Skin:** Dry, cool
- **Reflexes:** Delayed relaxation phase

### Workup
- **Laboratory Studies:**
  - TSH: 48 mIU/L (markedly elevated)
  - Free T4: 0.4 ng/dL (low)
  - Anti-TPO antibodies: Negative
- **Ultrasound Neck:** No thyroid tissue identified in normal cervical location
- **Thyroid Scintigraphy (I-123 or Tc-99m):** Uptake only at the base of tongue; no cervical thyroid tissue
- **CT Neck:** Confirms 2.5 cm mass at tongue base, no thyroid tissue in neck

### Diagnosis
**Lingual thyroid** (ectopic thyroid tissue at the base of tongue) with primary hypothyroidism

### Treatment
1. **Levothyroxine replacement therapy:**
   - Suppresses TSH
   - Reduces size of lingual thyroid through decreased stimulation
   - Treats hypothyroidism symptoms
2. **Monitor with TSH and imaging**
3. **Surgical excision** only if:
   - Severe obstructive symptoms not responding to suppression
   - Bleeding
   - Suspected malignancy (rare)
4. **If surgery required:** Lifelong levothyroxine as this may be only thyroid tissue

### Clinical Pearl
Lingual thyroid results from failure of the thyroid gland to descend during embryologic development, leaving functioning thyroid tissue at the foramen cecum. It is the most common location for ectopic thyroid tissue. Approximately 70% of patients with lingual thyroid have no other thyroid tissue and will become hypothyroid if the lingual thyroid is removed. This is why thyroid scintigraphy is essential before any surgical intervention - it confirms whether cervical thyroid tissue exists. Most patients can be managed conservatively with thyroid hormone suppression, which reduces TSH-driven growth of the ectopic tissue.

### Clinical Image
![Lingual Thyroid](case_02_image.jpg)

*Endoscopic view showing lingual thyroid as a smooth mass at the base of the tongue posterior to the circumvallate papillae.*

**Image Source:** Wikimedia Commons - "Lingual thyroid"
**License:** CC BY-SA 3.0
**URL:** https://commons.wikimedia.org/wiki/File:Lingual_thyroid.jpg

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## Case 3: Drug-Induced Thyroid Dysfunction (Amiodarone)

### Patient Demographics
- **Age:** 62 years
- **Sex:** Male
- **Occupation:** Retired banker

### Chief Complaint
"My cardiologist sent me because my thyroid tests are abnormal."

### History of Present Illness
A 62-year-old man with a history of atrial fibrillation has been on amiodarone 200 mg daily for 2 years with good arrhythmia control. Routine monitoring thyroid function tests performed last week were abnormal. He denies symptoms of hyperthyroidism or hypothyroidism - no palpitations, tremor, heat intolerance, weight changes, fatigue, or cold intolerance. He feels well overall and his atrial fibrillation has been controlled.

### Physical Examination
- **Vital Signs:** BP 128/78 mmHg, HR 72 bpm (regular)
- **General:** Well-appearing, no distress
- **Neck:** Thyroid normal size, non-tender, no nodules
- **Cardiovascular:** Regular rate and rhythm
- **Neurologic:** No tremor, reflexes normal

### Workup
- **Laboratory Studies:**
  - TSH: 6.8 mIU/L (elevated)
  - Free T4: 1.8 ng/dL (high-normal)
  - Free T3: 1.8 pg/mL (low-normal)
  - Total T4: 12.2 μg/dL (elevated)
  - Reverse T3: Elevated
- **Previous labs (before amiodarone):** TSH 2.4 mIU/L, Free T4 1.1 ng/dL

### Diagnosis
**Amiodarone-induced thyroid dysfunction** - early/subclinical amiodarone-induced hypothyroidism (AIH)

The lab pattern shows effects of amiodarone on thyroid physiology:
- Elevated T4 due to inhibition of peripheral T4-to-T3 conversion
- Relative decrease in T3
- Mildly elevated TSH due to decreased T3 at the pituitary level

### Treatment
1. **Continue amiodarone** if arrhythmia control is essential
2. **Close monitoring:** TSH and free T4 every 3-6 months
3. **If TSH continues to rise or symptoms develop:**
   - Levothyroxine supplementation while continuing amiodarone
   - Target TSH in normal range
4. **If amiodarone-induced thyrotoxicosis (AIT) develops:**
   - Type 1 AIT: Antithyroid drugs (iodine-rich gland)
   - Type 2 AIT: Glucocorticoids (destructive thyroiditis)
5. **Patient education** about symptoms of both hypo- and hyperthyroidism

### Clinical Pearl
Amiodarone contains 37% iodine by weight, and a 200 mg tablet delivers approximately 75 mg of iodine (far exceeding the 150 mcg daily requirement). Amiodarone affects thyroid function through multiple mechanisms: (1) inhibition of type 1 and type 2 deiodinases (blocks T4→T3 conversion), (2) high iodine load (can cause Wolff-Chaikoff effect or Jod-Basedow phenomenon), and (3) direct toxic effects on thyroid cells. Understanding thyroid hormone synthesis and metabolism is essential to interpreting these complex patterns. All patients on amiodarone require baseline and regular thyroid function monitoring (every 3-6 months), and amiodarone effects can persist for months after discontinuation due to its long half-life and tissue storage.

### Clinical Image
![Amiodarone Structure](case_03_image.jpg)

*Chemical structure of amiodarone highlighting its iodine-containing molecular structure and similarity to thyroid hormone.*

**Image Source:** Wikimedia Commons - "Amiodarone structure"
**License:** Public Domain
**URL:** https://commons.wikimedia.org/wiki/File:Amiodarone_structure.svg