This patient has postpartum thyroiditis, a condition that follows 5% to 8% of all pregnancies. Like other forms of destructive thyroiditis (including subacute or de Quervain thyroiditis), this illness is triphasic. Initially there is hyperthyroidism due to inflammation and release of preformed thyroid hormone from the inflamed follicles; this phase usually lasts 2 to 4 weeks. In subacute thyroiditis, the initial phase is usually noticed because of pain and tenderness over the thyroid gland, but in postpartum thyroiditis the thyroid is usually painless, and the hyperthyroid phase may be overlooked. This phase is then followed by transient hypothyroidism, usually lasting 1 to 3 months. The third phase is resolution and euthyroidism. Whereas Hashimoto thyroiditis usually leads to permanent autoimmune hypothyroidism, most patients with destructive thyroiditis have a full recovery. Some will be symptomatic enough to require thyroid supplementation for 1 to 3 months until the process resolves. Although the initial hyperthyroid phase can suggest Graves' disease, in thyroiditis the absence of infiltrative ophthalmopathy and a suppressed radioiodine uptake will make the distinction.Antithyroid drug treatment of thyroiditis is ineffective and puts the patient at unnecessary risk of toxicity such as agranulocytosis. Although hypothyroidism can cause amenorrhea and hence impair fertility, the hypothyroid phase of postpartum thyroiditis is transient. Low-level radiation exposure, but not thyroiditis, increases the risk of subsequent development of thyroid cancer. Interestingly, therapeutic radioactive iodine, such as is given for Graves' disease, does not increase the long-term risk of cancer, probably because the thyroid cells are destroyed.