T3 Toxicosis (T3 Thyrotoxicosis)
Your patient has T3 toxicosis—a form of hyperthyroidism characterized by elevated T3 (109 ng/dL, assuming units are ng/dL with normal ~80-200 ng/dL) with normal T4 (1.15 ng/dL) and suppressed/low-normal TSH (0.40 mIU/L), with negative TRAb (<1), indicating this is NOT Graves' disease.
Diagnosis
T3 toxicosis is confirmed when TSH is suppressed or low-normal with elevated free T3 but normal free T4, representing approximately 5-10% of all thyrotoxicosis cases 1, 2.
The negative TRAb (<1) effectively rules out Graves' disease, which accounts for 70% of hyperthyroidism cases but requires positive TSH-receptor antibodies for diagnosis 1, 3.
With negative TRAb, the differential diagnosis narrows to: toxic nodular goiter (16% of hyperthyroidism), toxic adenoma, subacute thyroiditis (3% of cases), drug-induced thyroiditis (9% of cases, particularly from amiodarone, tyrosine kinase inhibitors, or immune checkpoint inhibitors), or early/resolving thyroiditis 1, 2.
Diagnostic Workup Algorithm
Obtain thyroid ultrasound immediately to evaluate for nodules, as toxic nodular goiter is the second most common cause of hyperthyroidism after Graves' disease 1, 2.
Measure thyroid peroxidase (TPO) antibodies to evaluate for autoimmune thyroid disease, as positive antibodies suggest Hashimoto's thyroiditis in a thyrotoxic phase 4, 3.
Obtain radioactive iodine uptake scan (RAIU) if the etiology remains unclear after ultrasound: low/absent uptake indicates thyroiditis (self-limiting), while high uptake indicates autonomous thyroid tissue (toxic nodular goiter or toxic adenoma requiring definitive treatment) 4, 1, 2.
Review medication history specifically for amiodarone, immune checkpoint inhibitors (anti-PD-1/PD-L1, anti-CTLA-4), tyrosine kinase inhibitors, or recent iodine exposure (CT contrast), as these cause 9% of hyperthyroidism cases 5, 1.
Management Based on Etiology
If Thyroiditis (Low RAIU)
Conservative management during the thyrotoxic phase is sufficient—no antithyroid drugs are needed, as this is a self-limiting process 4.
Prescribe non-selective beta blockers (propranolol 20-40 mg three times daily or atenolol 25-50 mg daily) for symptomatic patients to control heart rate, tremor, and anxiety 5, 4.
Monitor for progression to hypothyroidism, which typically occurs 1 month after the thyrotoxic phase in thyroiditis cases, requiring initiation of levothyroxine replacement 4.
For patients on immune checkpoint inhibitors, continue immunotherapy in most cases—high-dose corticosteroids are rarely required for thyroid dysfunction 5, 4.
If Toxic Nodular Goiter or Toxic Adenoma (High RAIU)
Initiate methimazole (MMI) 10-20 mg daily as the preferred antithyroid drug to achieve euthyroidism before definitive treatment 2, 3.
Plan definitive treatment with radioactive iodine (RAI) ablation or thyroidectomy, as toxic nodular goiter rarely remits spontaneously and requires permanent treatment 1, 2.
Thyroidectomy should be performed by an experienced high-volume thyroid surgeon if surgery is chosen, with total thyroidectomy being the preferred approach 3, 6.
Radiofrequency ablation is rarely used but may be considered in select cases of toxic nodular goiter 1.
Critical Monitoring and Risk Assessment
Untreated hyperthyroidism causes cardiac arrhythmias (particularly atrial fibrillation), heart failure, osteoporosis, and is associated with increased mortality 1, 2.
Recheck TSH, free T4, and free T3 in 4-6 weeks after initiating treatment to evaluate response and adjust therapy accordingly 7.
For elderly patients or those with cardiac disease, more aggressive symptom control with beta blockers is essential, as prolonged TSH suppression (<0.1 mIU/L) increases risk for atrial fibrillation and cardiovascular mortality 4, 2.
Common Pitfalls to Avoid
Do not assume Graves' disease without positive TRAb, as negative antibodies exclude this diagnosis and change management entirely 1, 3.
Do not start antithyroid drugs for thyroiditis, as this is a destructive process releasing preformed hormone rather than increased synthesis, making antithyroid drugs ineffective 4, 1.
Do not delay RAIU scan if etiology is unclear, as distinguishing between thyroiditis (self-limiting) and autonomous thyroid tissue (requiring definitive treatment) fundamentally changes management 4, 1, 2.
For women of childbearing age, confirm pregnancy status before RAIU scan or RAI treatment, as both are contraindicated in pregnancy 3.