Subacute thyroiditis involves inflammation of the thyroid gland that usually follows an upper respiratory infection and then subsides.

Subacute thyroiditis is an uncommon condition thought to be caused by viral infection of the thyroid gland. The condition often occurs after a viral infection of the upper respiratory tract. Mumps virus, influenza virus, and other respiratory viruses have been found to cause subacute thyroiditis.

The most prominent feature of subacute thyroiditis is gradual or sudden onset of pain in the region of the thyroid gland. Painful enlargement of the thyroid gland may persist for weeks or months. The condition is sometimes associated with fever. Hoarseness or difficulty swallowing may also develop.

Symptoms of thyroid hormone excess (hyperthyroidism) such as nervousness, rapid heart rate, and heat intolerance may be present early in the disease. Later, symptoms of too little thyroid hormone (hypothyroidism) such as fatigue, constipation, or cold intolerance may occur. Eventually, thyroid gland function returns to normal.

Subacute thyroiditis occurs most often in middle-aged women with recent symptoms of viral respiratory tract infection.

• pain in the front of the neck
• tenderness when gentle pressure is applied to the thyroid gland (palpation)
• fever
• weakness
• fatigue

Other symptoms may include:

• nervousness
• heat intolerance
• weight loss
• sweating
• diarrhea
• tremor
• palpitations

Laboratory tests in the early phase of disease may reveal:

• High serum thyroglobulin level
• Low radioactive iodine uptake
• Low serum thyroid stimulating hormone (TSH) level
• High serum free T4 (thyroid hormone, thyroxine) level
• High erythrocyte sedimentation rate (ESR)

Laboratory tests in the later phase of disease may show:

• High serum TSH level
• Low serum free T4

Anti-thyroid antibodies are either undetectable or present at low levels. Thyroid gland biopsy shows characteristic "giant cell" inflammation. Laboratory abnormalities return to normal as the condition goes away.