Thyroid testing: should I be tested?

Hypothyroidism

An underactive thyroid can make you tired. You may also gain weight, feel cold, or have dry skin, thin hair, constipation or a hoarse voice. It is common, and one blood test, TSH, checks for it. If your TSH is normal, your thyroid is very unlikely to be the cause. These symptoms have many other causes. Extra thyroid tests, like free T3, reverse T3 or a 'full panel', usually add nothing when TSH is normal. They can also give confusing results. Some people should have TSH checked even without symptoms. This includes planning a pregnancy, having a baby in the last 6 months, past thyroid problems, or taking certain medicines. Tell your doctor about any swelling in your neck.

Hashimoto's thyroiditis with a normal TSH

Hashimoto's is the most common cause of an underactive thyroid. It is usually found because a blood test called TSH is high. Some people have a positive thyroid antibody test (TPO) while their TSH is normal. This means your immune system is reacting to your thyroid, but your thyroid is still working normally. A positive antibody with a normal TSH does not need treatment, and it does not explain tiredness on its own. It does mean you are more likely to get an underactive thyroid later, so checking TSH from time to time makes sense. If your TSH is also a little high, that is different. Your doctor may talk with you about treating sooner.

"Low T3" / reverse T3 dominance

Not a recognized medical diagnosis — here’s what it usually means and what to ask about instead.

You may have heard that your body is not 'converting' thyroid hormone well, or that a high reverse T3 is 'blocking' it. This is sometimes called 'low T3' or 'Wilson's syndrome'. It is not a recognized medical diagnosis, and the reverse T3 test has no proven use. Still, feeling unwell when your tests look normal is real and frustrating. If you take thyroid pills and still have symptoms with a normal TSH, talk with your doctor. Some research is looking at adding T3 for people in this situation, but so far it has not clearly helped most people. Testing reverse T3, or dosing thyroid pills by body temperature, is not recommended. It can also be unsafe.