Making the Diagnosis
If your doctor suspects hypothyroidism, tests can be performed to check how well the thyroid gland is functioning. Levels of T4 in the blood are measured, along with thyroid-stimulating hormone (TSH) that is produced by the pituitary gland. High levels of TSH suggest that the thyroid is underactive and that the pituitary is overcompensating by making excess hormones to stimulate the thyroid gland.
With secondary hypothyroidism, the pituitary gland isn't functioning properly and T4 levels in the blood are low. In this setting, TSH levels can be low or inappropriately normal.
Thyroid scans using radioactive iodine or technetium allow doctors to see the thyroid gland and identify areas that are less active. Thyroid ultrasound scans provide a detailed anatomic imaging of the gland, and can identify lesions (e.g. nodules, cysts), provide accurate size measurement, and show blood flow properties throughout the gland.