Approach to thyroid function testing

What it is

Thyroid testing runs in a fixed order. Define the clinical picture first, hypo, hyper or euthyroidal. Then look at the TFTs. Then screen the antibodies: anti-TPO, anti-TSHr and anti-TG. Thyroglobulin is kept for thyroidectomy patients, to rule out other causes of hyperthyroidism, since it will be suppressed. Then decide the trajectory, US neck against nuclear imaging, and nuclear imaging is only indicated in biochemically hyperthyroid patients. Then ask whether tissue sampling is needed. The final question is whether to treat and how: medical, radiation, or interventional and surgical.

How it is diagnosed

For a goiter, diffuse or nodular, measure TSH first.

• A high TSH sends you to free T4. A normal free T4 is subclinical hypothyroidism and a low free T4 is overt hypothyroidism; either way, measure TPO antibodies next. A positive TPO is Hashimoto's thyroiditis. A negative TPO with a high TSH is the seronegative form, or biosynthetic defects in thyroid hormone synthesis or iodine utilization, or an infiltrative disorder of the thyroid.
• A normal TSH also goes to TPO antibodies, and then to a thyroid ultrasound.
• A low TSH sends you to free T4 and total T3. Normal is subclinical hyperthyroidism; elevated is overt hyperthyroidism, the toxic goiter. Then determine the etiology, whether Graves' disease, multinodular goiter with autonomy, or painless thyroiditis, by measuring TRAb or by obtaining a thyroid uptake and scan, with a thyroid ultrasound that looks at thyroid vascularity.

If suspicious features are present, do a fine needle aspiration biopsy; if not, monitor. A thyroid ultrasound is not obtained routinely in Hashimoto's thyroiditis, and is reserved for larger goiters, thyroid asymmetry, or a concern for thyroid nodularity.