Thyroid Testing: TSH, Free T4, T3, rT3, and Anti-TPO#

TSH is made by the pituitary gland (in your brain), not the thyroid. The pituitary releases TSH as a signal telling the thyroid to make more thyroid hormone. When thyroid hormone is low, TSH rises; when thyroid hormone is high, TSH falls.

TSH is the most sensitive and reliable test for screening an underactive thyroid (hypothyroidism) or overactive thyroid (hyperthyroidism). Canadian guidelines recommend ordering TSH on its own as the first test, and only adding other thyroid tests if TSH is abnormal or if you have specific symptoms.

T4 is the main hormone the thyroid releases into the bloodstream. "Free" T4 is the small portion not bound to carrier proteins - the part that's biologically active. Free T4 is added to a TSH test when the TSH is abnormal, when symptoms are clearly present, or when central (pituitary) thyroid disease is suspected.

T3 is the more biologically active form of thyroid hormone. About 80% of T3 is made inside body tissues by converting T4 into T3 - only about 20% comes directly from the thyroid. T3 is usually only useful in specific situations (for example, a person with possible overactive thyroid and a normal T4). It is not routinely needed to diagnose an underactive thyroid.

Reverse T3 is a biologically inactive form of T3. There is no current Canadian guideline-based use for rT3 in diagnosing or monitoring thyroid disease. Rarely, rT3 is checked in critically ill hospital patients. TeleTest can add rT3 as a paid (uninsured) test on request, but our clinicians do not use it to guide care.

Anti-TPO antibodies are made by the immune system when it attacks the thyroid gland. A high level points to autoimmune thyroiditis (Hashimoto's thyroiditis), which is the most common cause of an underactive thyroid in Canada.

Anti-TPO is most useful when TSH is borderline (between 4 and 10 mIU/L) with a normal Free T4 - a positive result helps the clinician decide whether to start medication or monitor. Once anti-TPO is known to be positive, it does not need to be retested.

About 10% of people with Hashimoto's thyroiditis have negative anti-TPO antibodies, so a negative result does not fully rule out autoimmune thyroid disease.

Anti-thyroglobulin antibodies are another autoimmune marker that adds little beyond anti-TPO in most cases. Thyroid-stimulating immunoglobulin (TSI) is used to confirm Graves' disease (autoimmune overactive thyroid) and is usually ordered by an endocrinologist after referral.

• Feeling cold all the time
• Unusual tiredness lasting more than 3 months
• Slower thinking, "brain fog", trouble concentrating
• Constipation
• Dry skin
• Hair loss or hair thinning
• Unexpected weight gain
• Heavy or irregular periods
• Low mood
• Puffiness around the eyes and face

• Unintended weight loss
• Fast or irregular heartbeat, palpitations
• Feeling hot, sweating more than usual
• Tremor (shaky hands)
• Anxiety, irritability, trouble sleeping
• Diarrhea or more frequent stools
• Thinning skin, brittle hair
• Bulging eyes (in Graves' disease)

• You have already been diagnosed with thyroid disease and need routine monitoring
• You take thyroid hormone medication
• You take certain long-term medications that can affect thyroid function (such as some mood-stabilizer medications or some heart-rhythm medications) - monitor about every 6 months
• You have had head or neck radiation
• You have known pituitary or hypothalamus disease
• You are planning pregnancy, have had repeat miscarriages, or have a family history of thyroid disease
• You have other autoimmune diseases (such as type 1 diabetes, vitiligo, or celiac disease)

Canadian guidelines (including the Canadian Task Force on Preventive Health Care and Choosing Wisely Canada) do not recommend routine thyroid screening in people without symptoms or risk factors. If you have symptoms or any of the risk factors listed above, a screening TSH is appropriate.

No. Thyroid testing does not require fasting. You can have it done at any time of day, although TSH is slightly higher in the morning than in the afternoon, so consistency in timing helps when comparing results over time.

If you take thyroid hormone medication, take it at your usual time on test day. Some people prefer to delay the morning dose until after the blood draw - either approach is fine as long as you are consistent test-to-test.

• Biotin supplements (vitamin B7): stop biotin at least 72 hours before testing - it can falsely raise or lower thyroid results depending on the lab method.
• Severe illness, recent hospitalization, or acute infection: can cause temporarily abnormal results ("sick euthyroid syndrome"). Repeat testing once you have recovered.
• Pregnancy: normal ranges shift - see the pregnancy section below.

"Subclinical" means the TSH is abnormal but free T4 is still in the normal range. It often causes no symptoms. Whether to treat depends on how high (or low) the TSH is, whether you have symptoms, whether anti-TPO antibodies are positive, your age, and other conditions (such as heart disease or pregnancy planning).

Reference ranges change in pregnancy and are different in each trimester. A TSH between 2.5 and 4.0 mIU/L is reported as "normal" by most labs but may need treatment in pregnancy.

If you are pregnant or trying to become pregnant, we recommend you see your family doctor, midwife, or obstetrician for thyroid monitoring rather than using TeleTest, because pregnancy thyroid management requires close clinical follow-up.

• Autoimmune (Hashimoto's thyroiditis) - the most common cause in Canada
• Iodine deficiency - rare in Canada because table salt is iodized
• Medications - certain mood-stabilizer medications, certain heart-rhythm medications, immune-modulating cancer therapies, interferon
• After radiation to the head and neck
• After thyroid surgery
• Central hypothyroidism - rare; caused by pituitary or hypothalamus disease

• Graves' disease - the most common cause; an autoimmune condition where antibodies stimulate the thyroid
• Toxic nodular goitre - one or more nodules in the thyroid that make extra hormone
• Thyroiditis - temporary inflammation of the thyroid (after a viral illness, after pregnancy, or autoimmune)
• Too much thyroid hormone medication
• Iodine excess - rare; from supplements, certain heart-rhythm medications, or imaging contrast

This is called subclinical hypothyroidism. Most clinicians do not start medication right away if your TSH is between 4 and 10 mIU/L and your Free T4 is normal. The plan is usually to:

• Repeat the TSH (and Free T4) in 3 to 6 months to make sure it is not temporary
• Check anti-TPO antibodies - a positive result raises the chance the TSH will continue to climb
• Treat if you have clear symptoms, positive anti-TPO, heart disease, or you are pregnant or trying to conceive

A TSH above 10 mIU/L usually means clear hypothyroidism. Most clinicians will start thyroid hormone medication. The dose is then adjusted based on TSH measured every 6 to 8 weeks until stable, then yearly.

A low TSH suggests hyperthyroidism. The next step is a Free T4 (and often a Free T3) test to confirm. If overactive thyroid is confirmed, the typical next step is in-person specialist care with an endocrinologist - TeleTest does not manage overactive thyroid disease, because the workup often needs a thyroid uptake scan, ultrasound, and specialist follow-up. You would arrange the endocrinology referral through your family doctor or a walk-in clinic; TeleTest does not arrange endocrinology referrals.

Many people have positive anti-TPO antibodies without ever developing thyroid disease. A positive anti-TPO with normal TSH means a slightly higher lifetime risk of developing hypothyroidism. The usual plan is to recheck TSH every 1 to 2 years, or sooner if symptoms develop.

Many symptoms of an underactive thyroid (fatigue, weight changes, hair loss, dry skin) overlap with other common conditions: low iron, low vitamin D, sleep disorders, depression, perimenopause, and others. A normal TSH (especially in the lower half of the range) makes thyroid disease unlikely. Discuss with your clinician about other possible causes.

During serious illness, surgery, or hospitalization, thyroid hormone levels can shift temporarily even though the thyroid is healthy. TSH, T4, and T3 can all be abnormal. The recommended approach is to wait until you have recovered and repeat the test - it usually returns to normal.

Severe weight loss, fasting, and major life stress can cause small temporary shifts in thyroid hormone levels. Day-to-day stress generally does not cause clinically meaningful changes.

Generally no. T3 levels often remain normal even when someone has an underactive thyroid, so a T3 test is not useful for diagnosing hypothyroidism. T3 has a specific role in confirming overactive thyroid when T4 is borderline. Choosing Wisely Canada explicitly recommends against routine T3 testing.

No. There is no current Canadian guideline-based use for rT3 outside of intensive-care settings. It does not guide thyroid medication dosing, and most endocrinologists do not use it. TeleTest can add rT3 as a paid uninsured test on request, but our clinicians will not use it to guide your care.

TeleTest can write a renewal prescription for thyroid hormone medication if you have already been on a stable dose and have a recent TSH within range. New starts for thyroid hormone replacement and active dose adjustments for chronic thyroid disease are best done by a family doctor or endocrinologist, who can see you in person if the dose needs frequent changes.

• Eat a balanced diet with adequate iodine (iodized salt, dairy, eggs, seafood)
• Avoid extreme iodine supplementation (kelp, seaweed supplements) - too much iodine can trigger thyroid problems
• If you have selenium deficiency, correcting it may help autoimmune thyroid disease - but supplementation in well-nourished people is not generally helpful
• Stop smoking - smoking worsens autoimmune thyroid eye disease in Graves' disease
• If you take thyroid medication, take it consistently and apart from calcium, iron, and antacid supplements

• Normal result, no symptoms, no risk factors: retest only if new symptoms develop
• Subclinical hypothyroidism (TSH 4 to 10), being monitored: every 6 to 12 months
• On a stable dose of thyroid medication: once a year
• After a dose change: 6 to 8 weeks later (this is how long it takes for TSH to fully respond to a dose change)
• Pregnancy or planning pregnancy: under the care of your family doctor or OBGYN, not TeleTest
• On certain thyroid-affecting medications (some mood-stabilizers, some heart-rhythm medications): about every 6 months

In most provinces, TSH is covered when there is a clinical reason to test it (symptoms, known thyroid disease, certain medications, pregnancy planning). TeleTest only orders TSH as an insured test when there is a guideline-supported reason. If you want to monitor more often than guidelines recommend, you can pay an uninsured fee at the lab.

Free T4 is usually covered when added to an abnormal TSH or when ordered for clear clinical reasons. If you ask for Free T4 with a normal TSH for screening purposes, it may be billed as uninsured.

These are usually uninsured tests in most provinces unless there is a specific clinical reason (for example, anti-TPO when investigating subclinical hypothyroidism). The lab will charge you an extra fee at the time of your visit. TeleTest clinicians will only order these as insured tests when medically appropriate.

Provincial coverage rules apply consistently to TeleTest, and clinicians at TeleTest are subject to auditing by the provincial health authority. We follow what the guideline says and what the provincial plan will cover. Previous clinicians may have ordered tests as insured that should have been uninsured. We cannot speak to past decisions, but we work within the rules set by the Ministry of Health and provincial regulators.