Isolated Elevated T3 (>266 ng/dL) with Normal TSH and Normal T4

What is T3 Toxicosis?

• Elevated total T3 or free T3 (>200 ng/dL or above the lab's upper reference limit)
• Normal free T4
• Suppressed (low) TSH - the key finding

Important note: If the TSH in your report is also normal (not suppressed), that changes the interpretation - see below.

Two Scenarios Based on TSH:

Scenario A: T3 high + T4 normal + TSH suppressed (low)

• Occurs in about 5-10% of hyperthyroid patients
• The pituitary detects excess T3 and suppresses TSH - a "normal" TSH would be inappropriately normal in this setting
• Common causes:
  • Early Graves' disease (most common)
  • Toxic multinodular goiter
  • Toxic adenoma (Plummer syndrome)
  • Graves' disease + concurrent iodine deficiency (T3 preferentially secreted when iodine is scarce)
  • Relapse after antithyroid drug therapy

Scenario B: T3 high + T4 normal + TSH truly normal

• Pregnancy - increased T3 production, altered binding proteins (if measuring total T3)
• Recovery from thyrotoxicosis - T3 normalizes last
• Exogenous T3 ingestion - supplements or certain medications
• Non-thyroidal illness (sick euthyroid syndrome) - complex changes in T3/T4 conversion
• Elevated T3-binding proteins - estrogen therapy, oral contraceptives (if measuring total T3, not free T3)
• Rarely, TSH-secreting pituitary adenoma (but this would usually elevate T4 too)

Why Does T3 Rise Before T4?

What to Do Next?

Key Takeaway

• Quick Compendium of Clinical Pathology, 5th ed., p. 40
• Frameworks for Internal Medicine, p. 156
• Harrison's Principles of Internal Medicine, 22E
• American Thyroid Association - Thyroid Function Tests

Your Results = Classic T3 Toxicosis

What is happening?

1. Your thyroid is over-secreting T3 but not T4
2. The pituitary detects the excess T3, so it suppresses TSH (that is why your initial TSH was <0.10)
3. T4 remains normal because the thyroid is selectively producing more T3
4. T3 is the biologically active hormone - it is 3-4x more potent than T4

Important note on the "normal" TSH on the TFT panel: When T3 is elevated and causing symptoms, even a TSH that falls within the "normal range" on paper is considered inappropriately normal - the body should be suppressing TSH more. Your first test (TSH <0.10) is more reliable as a true indicator of hyperthyroid state. Lab timing differences can cause TSH to appear borderline.

Most Likely Causes (in order):

1. Early Graves' disease - most common cause, autoimmune, antibodies stimulate the thyroid
2. Toxic adenoma (a hyperactive nodule in the thyroid)
3. Toxic multinodular goiter (multiple overactive nodules)
4. Graves' disease + iodine deficiency (thyroid preferentially makes T3 when iodine is low)

Symptoms You May Have (or Watch For):

• Palpitations / fast heart rate
• Weight loss despite normal or increased appetite
• Feeling hot, sweating more than usual
• Trembling hands
• Anxiety, irritability, difficulty sleeping
• Loose stools / frequent bowel movements
• Tiredness despite feeling "wired"

What Should Be Done Next:

Bottom Line

• Rosen's Emergency Medicine, Table 117.2
• Harrison's Principles of Internal Medicine, 22E, p. 3069

Your Complete Clinical Picture - Putting It All Together

Your Symptoms vs. What Is Expected

The Weight Gain is the Key Atypical Feature

1. Toxic adenoma (a hyperactive nodule) - The nodule produces T3 independently. The rest of the thyroid may be suppressed. Some patients with toxic adenoma paradoxically retain or gain weight due to increased hunger/appetite compensating for metabolism.
2. Apathetic hyperthyroidism - Seen more in older patients - presents atypically without the "wired" feeling, and can have fatigue and weight gain rather than loss.
3. Concurrent condition alongside hyperthyroidism:
   • Hashimoto's thyroiditis (autoimmune) can coexist and cause a "swinging" state
   • Insulin resistance or metabolic syndrome causing weight gain while hyperthyroid symptoms are present
   • Fluid retention from another cause
4. Compensatory overeating - The body sends hunger signals due to metabolic demands, and if caloric intake exceeds expenditure, weight gain occurs despite hyperthyroidism.

The Lump on the Left Side of the Throat - THIS IS THE MOST URGENT FINDING

• Suppressed TSH (<0.10)
• Elevated T3
• Normal T4

• Producing T3 independently (hence isolated T3 elevation)
• Suppressing your TSH by negative feedback
• Causing all your hyperthyroid symptoms

However - a thyroid nodule MUST also be evaluated for malignancy (thyroid cancer)

"95% of all thyroid nodules are 'cold' (non-functional), with a risk of malignancy of 10-15%." - KJ Lee's Essential Otolaryngology

What You Need Urgently - Step by Step

Step 1: Thyroid Ultrasound (MOST URGENT - book this week)

• Will confirm the nodule, measure its size
• Assess features: Is it solid or cystic? Are margins regular or irregular? Any calcifications?
• Check lymph nodes around it
• This is the single most important next test

Step 2: Radioactive Iodine Uptake (RAIU) Scan with Thyroid Scintigraphy

• Will show if the nodule is "hot" (overactive - low cancer risk) or "cold" (low activity - higher cancer risk)
• Helps confirm toxic adenoma vs. other cause

Step 3: Blood Tests

• TRAb (TSH receptor antibodies) - rules in/out Graves' disease
• Anti-TPO antibodies - checks for Hashimoto's
• Calcitonin - rules out medullary thyroid cancer (rare but important with a nodule)
• Full blood count, liver function, blood glucose - hyperthyroidism affects these

Step 4: Fine Needle Aspiration (FNA) Biopsy - if ultrasound shows suspicious features

• Ultrasound-guided FNA is the gold standard for evaluating a thyroid nodule
• Recommended if nodule is ≥1 cm with any suspicious features

What Diagnosis to Expect

Toxic Adenoma of the Left Thyroid Lobe causing T3 Toxicosis

• Radioactive iodine (RAI) ablation - most common, destroys the overactive nodule
• Surgery (hemithyroidectomy) - removes the nodule, especially if large
• Antithyroid drugs (methimazole/carbimazole) - temporary control before definitive treatment
• Ethanol injection or radiofrequency ablation - in select cases

⚠️ Important Warning

• A neck lump you can feel
• Suppressed TSH
• Elevated T3

• Current Surgical Therapy 14e - Management of Hyperthyroidism
• KJ Lee's Essential Otolaryngology - Thyroid Nodule Evaluation
• Rosen's Emergency Medicine - Thyroid Function Test Interpretation

Please Tell Your Doctor About the Lump at Your Next Visit

A Palpable Neck Lump + Abnormal Thyroid Tests = A Different Clinical Situation

• Thyroid ultrasound - this cannot wait for multiple rounds of blood tests
• Assessment for malignancy - this is time-sensitive
• Possibly a biopsy (FNA)

What to Say to Your Doctor

"Doctor, I also noticed a lump on the left side of my neck/throat that I can feel. I wanted to mention it because I wasn't sure if it was related."

Why the Urgency?

Why Doctors Order a Second TFT

1. To Confirm the Abnormality is Real (Not a Lab Error)

• Sample handling issues
• Lab error
• Biotin supplements interfering with the assay (Harrison's specifically warns about this - biotin in multivitamins can cause falsely low TSH and falsely high T3)
• Timing of the blood draw (TSH has a slight daily rhythm - highest in early morning)

What the Second TFT Could Show - All Possibilities

The Most Likely Scenario in Your Case

• A palpable nodule (structural cause)
• Persistent symptoms (sweating, trembling, loose stools, poor sleep)
• Already confirmed suppressed TSH <0.10 and elevated T3

What Happens After the Second TFT

Confirmed abnormal TFT
        ↓
Thyroid Ultrasound  ←── This is where the lump becomes critical to mention
        ↓
TRAb antibodies (Graves' test)
        ↓
Radioactive iodine uptake scan
        ↓
Diagnosis + Treatment plan

One Practical Tip

• Get the blood drawn in the morning (TSH is most accurate then)
• Stop any biotin/multivitamin supplements at least 2-3 days before the test
• Fast or have only water if your lab recommends it
• Do it at the same lab as the first test if possible (consistent reference ranges)