LabMed

Toxic Multinodular Goiter (TMG)

At a Glance

Toxic multinodular goiter (TMG) is the second leading cause of hyperthyroidism after Grave’s disease in developed countries but may be the leading cause of hyperthyroidism in countries in which iodine deficiency is common. Iodine deficiency can lead to a goiter formation with repeated cycles of involution and hyperplasia, subsequently resulting in nodule formation. In this context, toxic nodules can arise and produce elevated levels of T4. The presenting symptoms of TMG may be the same as the presenting symptoms for hyperthyroidism: heat intolerance, weight loss, increased appetite, nodular goiter, tremor, and irritability. Clinical hyperthyroidism tends to manifest once nodules exceed 2.5 cm in their greatest dimension.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The laboratory findings of TMG are that of hyperthyroidism. Generally, a diagnosis of hyperthyroidism is made when a pattern of low TSH and elevated fT4 and/or TT4 is seen. Still, up to one-half of patients with toxic nodules show an elevated T3 but normal T4. These findings may be taken into consideration along with imaging studies of radioactive iodine uptake.(Table 1)

Table 1

Tests Results Indicative of Toxic Multinodular Goiter
TSH Free T4 Total T3
<0.1 mclUnits/mL >1.8 ng/dL >181 ng/dL

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Hospitalized patients may have transiently low or high TSH. Most frequently, TSH levels are suppressed during the acute phase of illness or during treatment on glucocorticoid or dopamine therapy. Other drugs like amiodarone can increase TSH levels. Critically ill euthyroid patients may be differentiated from hyperthyroid ill patients, because the latter show profoundly low TSH values less than 0.01 mU/L. Increases in T3 and T4 may occur with the ingestion of large quantities of exogenous thyroid hormone.

What Lab Results Are Absolutely Confirmatory?

Although no laboratory test is absolutely confirmatory for TMG, the pattern of low TSH and elevated T3/T4 suggests hyperthyroidism and, in the presence of clinically apparent enlarged gland, is suggestive of TMG. These findings may be taken into consideration along with imaging studies of radioactive iodine uptake.

What Tests Should I Request to Confirm My Clinical Dx? In addition, what follow-up tests might be useful?

The errors in test interpretation for TMG are likewise by extension the errors in test interpretation for hyperthyroidism. The clinical setting must be considered when interpreting laboratory tests for hyperthyroidism. As mentioned, suppression of TSH may occur in ill, hospitalized patients, and pharmacotherapeutics can inflate TSH levels. Failure to treat TMG can result in ongoing symptoms of hyperthyroidism and long-term sequelae.

Are There Any Factors That Might Affect the Lab Results? In particular, does your patient take any medications - OTC drugs or Herbals - that might affect the lab results?

Amiodarone can increase TSH levels. Critically ill euthyroid patients may be differentiated from hyperthyroid ill patients, because the latter show profoundly low TSH values less than 0.01 mU/L. Increases in T3 and T4 may occur with the ingestion of large quantities of exogenous thyroid hormone further obscuring the picture of TMG.

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