How do I know if my thyroid is overactive?

An overactive thyroid, or thyrotoxicosis, speeds up systems that hypothyroidism slows down. The National Institute of Diabetes and Digestive and Kidney Diseases lists weight loss despite an increased appetite, rapid heartbeat, nervousness, tremor, excessive sweating, fatigue, muscle weakness, frequent bowel movements and goitre. Symptoms often build over weeks rather than months, and the cardiac ones bring most people in.

How an overactive thyroid is confirmed

The 2016 American Thyroid Association hyperthyroidism guideline recommends that serum TSH, free T4 and total T3 all be assessed at the initial evaluation of suspected thyrotoxicosis. TSH is suppressed first. Free T4, total T3 or both are then raised in overt disease. Where TSH is low but free T4 and T3 sit within their normal ranges, the picture is subclinical hyperthyroidism, which the same guideline defines exactly that way. Biotin supplements can create a false Graves' pattern on these assays, so they should be stopped before testing.

Finding the cause

Cause matters because treatment differs completely. Graves' disease is autoimmune and is confirmed by a positive TSH receptor antibody, which the 2016 guideline describes as cost effective because a positive result establishes the most common cause without further imaging. A toxic nodule or toxic multinodular goitre produces autonomous hormone output and is identified on a radionuclide uptake scan. Thyroiditis releases stored hormone from a damaged gland, shows low uptake, is usually self-limiting and does not respond to antithyroid drugs. Taking too much thyroid hormone, including hormone hidden in supplements, produces the same blood picture.

The three treatment routes

For Graves' disease there are antithyroid drugs, radioiodine and thyroidectomy. Methimazole is the preferred antithyroid drug outside the first trimester of pregnancy, where propylthiouracil is used instead. Anyone starting an antithyroid drug must be told about agranulocytosis: the 2016 guideline instructs that patients report fever, sore throat or mouth ulcers immediately and stop the drug pending a white cell count.

If you have palpitations with weight loss and heat intolerance, ask for TSH, free T4 and total T3 on the same draw, and bring every supplement bottle to the appointment. Untreated thyrotoxicosis causes atrial fibrillation and bone loss, so this is one thyroid problem that should not be watched indefinitely.