Toxic Multinodular Goiter
The most likely diagnosis is toxic multinodular goiter (TMNG). The combination of hyperthyroidism (suppressed TSH, elevated free T4), a bilaterally enlarged non-tender thyroid gland, and a radioactive iodine uptake scan showing increased uptake in multiple thyroid nodules is pathognomonic for TMNG 1.
Diagnostic Reasoning
The clinical presentation and diagnostic workup clearly distinguish TMNG from the other options:
Why TMNG (Option d):
- Radioiodine uptake pattern is definitive: The scan demonstrates "multiple areas of focal increased and suppressed uptake" which is the hallmark imaging finding for TMNG 1. This represents multiple autonomously functioning nodules with suppression of intervening normal thyroid tissue.
- Age and presentation fit: At 57 years old, this patient is in the typical demographic for TMNG, which predominantly affects middle-aged to elderly individuals 2, 3.
- Biochemical profile matches: Suppressed TSH (0.15) with elevated free T4 (4.0) indicates overt hyperthyroidism with normal or elevated radioiodine uptake, consistent with toxic nodular goiter 1.
Why NOT Graves' Disease (Option a):
- Uptake pattern is wrong: Graves' disease shows diffuse increased uptake throughout the thyroid gland, not multiple focal areas 1. The guideline explicitly states "The pattern of RAIU in GD is diffuse unless there are coexistent nodules or fibrosis" 1.
- Missing clinical features: No ophthalmopathy, no recent onset of eye findings, and the patient lacks the typical presentation that would make Graves' disease "sufficiently likely" without further testing 1.
Why NOT Papillary Thyroid Cancer (Option b):
- Cancer doesn't cause hyperthyroidism: Thyroid malignancies typically present as euthyroid nodules. The guidelines specifically address that cancer prevalence in TMNG is approximately 3-9% 1, but the cancer itself doesn't cause the hyperthyroidism—the autonomous nodules do.
- Uptake pattern excludes malignancy as primary diagnosis: Functioning nodules showing increased uptake are typically benign adenomas, not malignant lesions.
Why NOT Fibrous Thyroiditis (Option c):
- Uptake pattern is opposite: Thyroiditis (whether subacute, painless, or postpartum) presents with near-zero radioiodine uptake due to thyroid destruction and hormone release, not increased uptake 1. The guideline clearly categorizes thyroiditis under "Thyrotoxicosis associated with a near-absent radioiodine uptake over the neck" 1.
- Clinical features don't match: The thyroid is non-tender (excluding subacute thyroiditis) and the chronic bilateral enlargement with nodularity doesn't fit any thyroiditis pattern.
Clinical Implications
This 57-year-old woman requires immediate management given her cardiovascular symptoms (tachycardia, hypotension) and the risk of complications:
- Beta-blockade is essential: Given her age >60 years and cardiovascular symptoms (tachycardia, low blood pressure), beta-blockers should be initiated immediately to prevent post-treatment tachyarrhythmias 1.
- Consider methimazole pretreatment: As an elderly patient with cardiovascular manifestations, she meets criteria for methimazole pretreatment before definitive radioactive iodine therapy 1.
- Evaluate for malignancy: Before selecting radioactive iodine as definitive treatment, any nonfunctioning or suspicious nodules should be evaluated per thyroid nodule guidelines, as cancer prevalence in TMNG can reach 9% 1.
The radioiodine uptake scan showing multiple hot nodules definitively establishes TMNG as the diagnosis, making option d correct.