Diagnosis: Toxic Nodular Goiter
The most likely diagnosis is C. Toxic nodular goiter (toxic multinodular goiter), given the classic presentation of thyrotoxicosis with a non-tender goiter and negative thyroid-stimulating immunoglobulins, which effectively rules out Graves' disease. 1
Clinical Reasoning
Why Toxic Nodular Goiter is Most Likely
The patient presents with classic thyrotoxicosis symptoms (weight loss, heat intolerance, tremor, irritability) combined with a non-tender goiter and negative TSI antibodies—this constellation strongly points to toxic nodular goiter rather than Graves' disease. 1
- Toxic multinodular goiter is one of the most common causes of thyrotoxicosis after Graves' disease, and presents with symptoms of hyperthyroidism including heat intolerance, weight loss, tremor, and anxiety. 1
- The negative thyroid-stimulating immunoglobulins (TSH receptor antibodies) is the critical distinguishing feature that excludes Graves' disease, as Graves' disease is diagnosed by the presence of TSH receptor antibodies. 1
- Toxic nodular goiter typically develops in patients with longstanding multinodular goiter (often >5 years), where nodules develop autonomous function independent of TSH stimulation. 2
Why Other Options Are Incorrect
A. Primary TSH-secreting tumor would present with elevated TSH levels (not suppressed) along with elevated thyroid hormones—a rare condition that contradicts the typical biochemical pattern of thyrotoxicosis where TSH is suppressed. 3
B. Tertiary hypothyroidism would cause hypothyroid symptoms (weight gain, cold intolerance, constipation), not hyperthyroid symptoms, making this diagnosis incompatible with the clinical presentation. 1
D. Graves' disease is effectively ruled out by the negative thyroid-stimulating immunoglobulins, as Graves' disease is characterized by positive TSH receptor antibodies. 1 While Graves' disease is the most common cause of hyperthyroidism globally (2% prevalence in women), the negative antibody test excludes this diagnosis. 3
Diagnostic Confirmation
To confirm toxic nodular goiter, the following would be expected:
- Radioactive iodine uptake and scan would show patchy uptake with "hot" nodules (areas of autonomous function) and suppressed uptake in surrounding tissue. 1
- Thyroid ultrasound would reveal multiple nodules within the goiter and help evaluate for any suspicious features. 1
- Suppressed TSH with elevated free T4 and/or T3 levels would confirm overt hyperthyroidism. 3
Clinical Pitfalls to Avoid
- Do not assume all thyrotoxicosis with goiter is Graves' disease—always check TSH receptor antibodies to distinguish between Graves' disease and toxic nodular goiter. 1
- In patients with nodular goiter and thyrotoxicosis, consider the possibility of concurrent thyroid malignancy, as the incidence of carcinoma can be elevated in multinodular goiters (approximately 4% in some series). 2, 4
- A non-tender goiter helps distinguish this from subacute thyroiditis, which typically presents with a tender thyroid gland. 1