LabMed

Subacute Thyroiditis

At a Glance

Subacute thyroiditis is a biphasic entity that can present as diffuse thyroid inflammation, swelling, and hyperthyroidism in the early phase of the disease. The presenting symptoms of hyperthyroidism include heat intolerance, weight loss, diarrhea, increased appetite, tremor, and irritability. This hyperthyroid period is followed by a late phase of hypothyroidism with opposing clinical symptoms like cold intolerance, constipation, and lack of energy. In the vast majority of cases, this hypothyroid phase is followed by a resolution of symptoms and a return to the euthyroid state.

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

Laboratory tests for the assessment of a patient with suspected subacute thyroiditis include thyroid-stimulating hormone (TSH), free thyroxine (fT4), and total triiodothyronine (T3). TSH is a glycoprotein hormone with an alpha and a beta subunit. TSH is secreted by the anterior pituitary gland as a result of a negative feedback loop involving T3 and fT4. TSH is suppressed during the hyperthyroid (early) phase of subacute thyroiditis, as it is in all hyperthyroid patients except those with a TSH-secreting pituitary tumor. A diagnosis of hyperthyroidism can be made with a pattern of low TSH and elevated fT4 and/or T3. The hypothyroid (late) phase of subacute thyroiditis is suggested by the reverse pattern of high TSH and low fT4 and/or T3 (Table 1).

Table 1.

Test Results Indicative of the Disorder
TSH total T3 free T4
Early <0.1 mcIUnits/mL >181 ng/dL >1.8 ng/dL
Late >5.0 mclUnits/mL <80 ng/dL <0.8 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. TSH can be suppressed during glucocorticoid or dopamine therapy, whereas other drugs like amiodorone can increase TSH levels. Increases in T3 and fT4 may occur with ingestion of large quantities of exogenous thyroid hormone.

What Lab Results Are Absolutely Confirmatory?

Although no single laboratory test is absolutely confirmatory for subacute thyroiditis, the pattern of low TSH and elevated fT4 and/or T3 suggests hyperthyroidism, and the reverse pattern suggests hypothyroidism, perhaps pointing to either early or late phases of subacute thyroiditis, respectively. These findings should be taken into consideration along with other aspects of the clinical history such as recent viral illness or findings suggestive of another specific thyroid disease, such as Grave’s disease.

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 subacute thyroiditis are likewise by extension the errors in test interpretation for hyper- and hypothyroidism. The clinical setting must be considered when interpreting any laboratory tests. As mentioned, suppression of TSH may occur in ill, hospitalized patients and pharmacotherapeutics can inflate TSH levels.

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. Increases in T3 and T4 may occur with the ingestion of large quantities of exogenous thyroid hormone.

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