At a Glance
Painless thyroiditis (silent thyroiditis) is characterized by the transient onset of symptoms of thyrotoxicosis, followed by a resolving hypothyroidism. Pain and tenderness of the thyroid are usually absent. Any symptoms of thyrotoxicosis, such as jitteriness, increased perspiration, easy fatigability, heat intolerance, tachycardia, and muscle weakness, can be present. Usually, symptoms are mild and most patients with silent thyroiditis will likely consult a family practitioner, rather than an endocrinologist. Among the signs, mild enlargement of the thyroid may be present, but infiltrative orbitopathy is absent.
Silent thyroiditis is often grouped with post-partum thyroiditis, which by definition occurs within the first year of delivery. These disorders are also known as destruction-induced thyrotoxicosis, since they are caused by a rapidly progressive tissue injury associated with the release of thyroid hormones into circulation followed by a period of hypothyroidism, with subsequent complete resolution in most cases. These episodes last about 4-6 weeks in duration, with a complete recovery in approximately 3 months. Recurrences are rare. Residual hypothyroidism is more common in post-partum thyroiditis.
What tests should I request to confirm my clinical Dx? In addition, what follow-up tests might be useful?
Silent thyroiditis (destruction-induced thyrotoxicosis) must be distinguished from stimulation-induced thyrotoxicosis seen in Grave’s disease. Grave’s disease is treated with antithyroid drugs, radioisotope therapy, or surgical subtotal thyroidectomy. Silent thyroiditis can usually be treated conservatively. Subacute thyroiditis can be excluded by its association with a painful goiter.
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A starting panel of tests should include Total T3, T4, FT4, and TSH. If hyperthyroidism is detected (elevated T3,T4, FT4, low TSH), radioactive iodine uptake (RAIU) study along with scintigraphy is indicated. A T3/T4 ratio (ng/µg) less than 20 is associated with destruction-induced thyrotoxicosis, and a ratio greater than 20 is associated with Grave’s disease.(Table 1)
Table 1.
| TSH | FT4 | T3/T4 Ratio |
|---|---|---|
| low | high | <20 |
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 lab results?
The T3/T4 ratio can be affected by Thyroxine-binding Globulin (TBG) levels, which can increase in pregnancy and in those using oral contraceptives. Use of the FT3/FT4 ratio, which is not affected by TBG levels, is theoretically appealing but has not been found useful in differentiating silent thyroiditis from Grave’s disease.
What Lab Results are Absolutely Confirmatory?
Thyroid scintigraphic scans and imaging can be done with Technetium (Tc)-99m pertechnetate or with I-123 at the request of the clinician. I-123 is commonly used for radioisotope uptake studies, but is more expensive. Thyroid scintigraphy with Tc-99m reveals markedly reduced glandular activity. RAIU studies reveal markedly reduced uptake of I-123. In contrast, the uptake and glandular activity is markedly increased in Grave’s disease.
Additional Issues of Clinical Importance
Thyroid biopsy is not usually done in silent thyroiditis but shows a marked lymphocytic infiltration of the thyroid follicles. The histologic findings can be difficult to distinguish from those in Hashimoto’s thyroiditis.
Errors in Test Selection and Interpretation
Thyroid scintigraphic scans and imaging can be done with Technetium (Tc)-99m pertechnetate or with I-123 at the request of the clinician. Tc-99m is preferred, because the allowable dose of Tc-99m is higher than that for I-123, and, therefore, the scan data can be obtained more quickly. In general, anti-TSH antibodies, Thyroid Peroxidase Antibodies (TPO), and Thyroglobulin levels are not specific enough for the use in silent thyroiditis.
Anti-TSH receptor antibodies (TBII) can be used to diagnose Grave’s disease, but 10-30% of patients with Grave’s disease are negative for TBII, whereas 15% of silent thyroiditis cases are positive for TBII. Factitious thyrotoxicosis (due to ingestion of thyroxine) can be a challenge to physicians and, if suspected, should be considered in those with a low RAIU scan. The serum thyroglobulin concentration is usually increased in silent thyroiditis but is likely low in factitious thyrotoxicosis.
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