Rapid Growth of Euthyroid Multinodular Goiter: Causes

1. Thyroid Lymphoma (Most Important Differential)

• Thyroid lymphoma often arises in the background of Hashimoto's thyroiditis (chronic autoimmune thyroiditis). Chronic antigenic lymphocyte stimulation leads to lymphocyte transformation.
• The classic presentation is a rapidly expanding, often painless thyroid mass in a patient who may be euthyroid, though many are already hypothyroid or on thyroid replacement.
• Diffuse large B-cell lymphoma (DLBCL) is the most common type.
• Chronic thyroiditis increases the risk of primary thyroid lymphoma by 70 to 80 fold; in one series, 90% of patients with primary thyroid lymphoma also had Hashimoto's thyroiditis.
• Annual incidence is only 2 per 1 million, but a rapidly enlarging mass in a thyroid gland with pre-existing inflammation is highly suspicious.
• Diagnosis: FNAB (though core biopsy may be needed as lymphoid cells can be hard to distinguish cytologically); full staging workup to exclude systemic lymphoma.
• Key point from Harrison's (2025): "A rapidly expanding thyroid mass suggests the possibility of this diagnosis."
• Treatment: Chemotherapy (CHOP regimen), radiation - NOT surgery as the primary modality.

• Harrison's Principles of Internal Medicine 22E, p. 764
• Cummings Otolaryngology, p. 130
• K.J. Lee's Essential Otolaryngology, p. 2860

2. Anaplastic Thyroid Carcinoma (ATC)

• ATC is the most aggressive thyroid malignancy and one of the fastest-growing solid tumors known.
• It can arise de novo or from transformation of a pre-existing differentiated thyroid cancer or MNG.
• Despite explosive growth, patients are typically euthyroid because these undifferentiated cells do not produce thyroid hormone.
• Usually presents in older patients (>60 years), and growth can be dramatic over weeks to months.
• Prognosis is extremely poor - most patients die within 6 months of diagnosis.
• Associated symptoms: dysphagia, hoarseness, dyspnea (compressive symptoms), but thyroid function tests remain normal.

• Harrison's Principles of Internal Medicine 22E, p. 757-759

3. Hemorrhage into a Thyroid Nodule (Cystic Degeneration)

• A cystic or partially cystic nodule within a pre-existing MNG can undergo sudden intranodular hemorrhage.
• This can cause rapid and dramatic enlargement (sometimes overnight), along with pain and tenderness.
• Thyroid function remains normal (euthyroid).
• This is a benign cause of rapid enlargement, though it can be alarming.
• Diagnosis: Ultrasound will show a complex or predominantly cystic lesion with internal echoes consistent with hemorrhage; confirmed by FNAB.
• Harrison's lists "hemorrhage into a cyst" explicitly in the differential for sudden thyroid enlargement.

• Harrison's Principles of Internal Medicine 22E, p. 293

4. Malignant Transformation within MNG (Papillary or Follicular Thyroid Cancer)

• A pre-existing MNG can harbor a focus of papillary thyroid carcinoma (PTC) or follicular carcinoma, which grows within the goiter.
• These tumors are well-differentiated and do not disrupt thyroid hormone production, so the patient stays euthyroid.
• Rapid growth of one nodule within the MNG, rather than the whole gland, is a warning sign.
• Hashimoto's thyroiditis in the background is associated with increased risk of PTC.

• Sabiston Textbook of Surgery, p. 3099-3111
• Family Medicine Textbook (diagnostic flowchart for thyroid nodules)

5. Riedel's Thyroiditis (Fibrous Thyroiditis)

• A rare, chronic fibrosing process where the thyroid is replaced by dense fibrous tissue that invades surrounding structures.
• Can cause rapid, rock-hard enlargement mimicking malignancy.
• Euthyroid in early stages; hypothyroid later.
• Often associated with other fibrosclerotic conditions (retroperitoneal fibrosis, sclerosing cholangitis).

6. Iodine Deficiency / Environmental Goitrogens (Less Likely for This Rate)

• Chronic iodine deficiency or goitrogen exposure can cause progressive MNG enlargement, but at a much slower pace - this would not explain a 4-fold increase in 6 months.
• Mentioned for completeness as a background etiology of MNG.

Clinical Approach: What to Do Next

Why She Has No Symptoms of Hypo- or Hyperthyroidism

Reason 1: The Growing Mass Is Not Functional Thyroid Tissue

• Thyroid Lymphoma: The rapidly proliferating cells are lymphocytes, not thyroid follicular cells. Lymphocytes do not make T3 or T4. The remaining normal thyroid tissue continues to produce hormone at a normal rate. So TSH stays normal, the patient stays euthyroid, and there are no thyroid hormone symptoms whatsoever. The gland is simply being taken over by a non-secreting mass.
• Anaplastic Thyroid Carcinoma (ATC): These are undifferentiated cells that have lost all normal thyroid follicular cell function. They do not synthesize thyroglobulin or secrete T3/T4. Again, the mass grows purely as structural bulk, while whatever functional tissue remains keeps hormone levels in the normal range.
• Hemorrhage into a nodule: The blood filling the nodule has no endocrine activity. The cyst simply expands mechanically.

Reason 2: The "Nontoxic" MNG Phenomenon

• Nodules grow at different rates, some are hyperfunctioning and some are non-functioning, and they cancel each other out in terms of net hormone production.
• The HPT axis (hypothalamus-pituitary-thyroid axis) is intact - if one group of cells over-produces hormone, TSH is suppressed and the rest of the gland compensates down. The feedback loop maintains euthyroid status even as the architecture becomes increasingly nodular and disordered.
• As Guyton & Hall explains, even in colloid goiter where the gland grows enormously, "these goitrous glands may secrete normal quantities of thyroid hormones."

• Harrison's Principles of Internal Medicine 22E, p. 442
• Guyton & Hall Medical Physiology, p. 1666

Reason 3: TSH Sensitivity Is the Gatekeeper

• TSH may be normal or even slightly low (due to subclinical autonomy in some nodules).
• The nodules have acquired TSH-independent growth via somatic mutations (TSHR mutations, Ras mutations) - they grow without needing more TSH, but they do not overproduce hormone to a degree that overwhelms the feedback axis.
• When hormone levels drift up slightly, the pituitary suppresses TSH, which brakes the functional output of the remaining normal tissue - keeping the patient euthyroid.

Summary Table