What is the diagnosis and workup for a 27-year-old female with a mid-normal Thyroid-Stimulating Hormone (TSH) level, normal free Thyroxine (T4), and elevated free Triiodothyronine (T3)?

Diagnostic Approach to Elevated Free T3 with Normal TSH and Free T4

A patient with normal TSH, normal free T4, and elevated free T3 should undergo evaluation for T3 thyrotoxicosis, with thyroid antibody testing and thyroid imaging as the next diagnostic steps. This biochemical pattern suggests selective T3 elevation that requires thorough investigation to determine the underlying cause.

Initial Assessment

When evaluating a 27-year-old female with this specific thyroid function pattern, consider:

• Confirm the laboratory abnormality with repeat testing to rule out laboratory error
• Assess for symptoms of thyrotoxicosis (weight loss, heat intolerance, palpitations, anxiety, tremors)
• Evaluate for physical signs of thyroid disease (goiter, nodules, tachycardia)

Differential Diagnosis

The most likely diagnoses to consider include:

1. T3 Thyrotoxicosis - A form of hyperthyroidism where primarily T3 is elevated while T4 remains normal 1
2. Early/Mild Graves' Disease - Can sometimes present with isolated T3 elevation before affecting T4 2
3. Autonomous Functioning Thyroid Nodule - Can produce predominantly T3 1
4. Multinodular Goiter - May have areas of autonomous function producing excess T3 1
5. TSH-secreting Pituitary Adenoma - Rare but should be considered when TSH is inappropriately normal with elevated thyroid hormones 3

Recommended Diagnostic Workup

1. Thyroid Antibody Testing:

   • Thyroid stimulating immunoglobulin (TSI) or TSH receptor antibody (TRAb) to evaluate for Graves' disease
   • Thyroid peroxidase (TPO) antibodies to assess for autoimmune thyroid disease 2

2. Thyroid Imaging:

   • Thyroid ultrasound to evaluate for nodules or goiter
   • Radioactive iodine uptake scan (RAIUS) or Technetium-99m thyroid scan to assess for areas of increased uptake suggesting autonomous function 2, 1

3. Additional Laboratory Tests:

   • Alpha subunit of TSH if suspecting a TSH-secreting pituitary adenoma
   • Repeat thyroid function tests in 4-6 weeks to assess for progression or resolution 1

4. Consider Pituitary Imaging:

   • MRI of the pituitary if TSH remains inappropriately normal with persistent thyroid hormone elevation to rule out TSH-secreting adenoma 3

Management Considerations

Management will depend on the underlying diagnosis:

• For T3 thyrotoxicosis or Graves' disease: Consider anti-thyroid medications, radioactive iodine treatment, or surgery depending on severity and patient factors
• For autonomous nodules: Radioactive iodine treatment or surgical removal may be indicated 1
• For subclinical disease: Close monitoring may be appropriate if the patient is asymptomatic

Important Caveats

• Isolated T3 elevation with normal TSH is uncommon and warrants thorough investigation
• Some patients with this pattern may progress to overt hyperthyroidism over time 1
• Non-thyroidal illness can cause abnormal thyroid function tests but typically presents with low T3, not elevated T3 4
• Laboratory interference should be considered if the clinical picture doesn't match the biochemical findings
• Repeat testing is essential as transient elevations can occur

This pattern of thyroid function tests requires careful evaluation as it may represent early thyroid disease that could progress and impact the patient's health if not properly diagnosed and managed.