Your doctor will typically run several blood tests to diagnose postpartum thyroiditis. In the hyperthyroid phase, your blood tests typically show a low thyroid-stimulating hormone (TSH), and high-normal or elevated thyroxine (T4) and triiodothyronine (T3).
In the hypothyroid phase, your TSH will be elevated, and T4 and T3 will be low or low-normal. Thyroid peroxidase (TPO) antibody levels are likely to be elevated in the majority of women with postpartum thyroiditis, especially during the hypothyroid phase.
In some cases of postpartum thyroiditis, an ultrasound is performed and will show enlargement of your thyroid gland.
It's important to note that along with postpartum thyroiditis, autoimmune Graves' disease (which causes hyperthyroidism) may occur after your baby is born. While postpartum thyroiditis is a far more common cause of hyperthyroidism, your doctor will want to ensure he does not miss a diagnosis of Graves' disease.
Some distinguishing factors are that in Graves' disease, a woman has more severe symptoms, more thyroid enlargement, and may have eye-related symptoms (called Graves' ophthalmopathy).
In some cases, a radioiodine uptake test is done to differentiate postpartum thyroiditis from Graves' disease. (Note, however, this test is not performed if you are breastfeeding.)