The Thyroid: TSH, T3, T4, rT3

What role does thyroid hormone play in the body?

Thyroid hormone (T4 or thyroxine) is produced by the thyroid gland that sits just below the Adam’s Apple or larynx. Thyroid hormone plays a key role in the body’s metabolic functions and has important effects on the central nervous system, and cardiac and reproductive systems. The thyroid gland releases thyroid hormone in response to a signal from the pituitary gland called Thyroid Stimulating Hormone (TSH). Excessive thyroid hormone or too little thyroid hormone can manifest through different signs and symptoms described below. The key thyroid-related hormones are Thyroid Stimulating Hormone (TSH), Thyroxine (T4), and Triiodothyronine (T3).

When should I check my TSH thyroid?
  • You have had symptoms of hypothyroidism or hyperthyroidism

  • You have been diagnosed with hypothyroidism or hyperthyroidism, or had thyroid surgery

  • You are currently on lithium or amiodarone (monitor every 6 months)

  • You have had a history of head and neck irradiation

  • You have a medical disease of the hypothalamus or pituitary gland (i.e. pituitary adenoma, prolactinoma, craniopharyngioma, etc.)

What is an 'uninsured' test?

Uninsured lab tests are tests that are not medically necessary to manage a medical disease or condition. For example, T3, T4 and rT3 levels are not used to monitor therapy in the treatment of hypothyroidism (your TSH is), so these tests are not medically 'indicated'. You can elect to add these tests to your bloodwork, but OHIP will not pay for these tests and you must pay additional fees at the time you visit the lab. TeleTest physicians will only order testing as insured when medically appropriate.

In the past, you may have found that your family doctor or walk-in clinic ordered these tests as insured tests. We cannot speak to prior circumstance, but presently, we're required to follow the guidance from the Ministry of Health and the College of Physicians and Surgeons of Ontario.

How do I interpret my lab tests?


TSH (0.4-4 mIU/L) *

Free T4

Do I need medication?

Frequency of Repeat Testing

Normal Thyroid Function




Not indicated

Sub-Clinical Hypothyroidism



No, unless **

6-12 months or sooner if symptoms develop

Sub-Clinical Hypothyroidism

10 +



Every 6 weeks with medication dose changes, then annually when stable

Primary Hypothyroidism




Every 6 weeks with medication dose changes, then annually when stable

Primary Hyperthyroidism




*Defer to endocrinology

Sub-clinical Hyperthryoidism




*Defer to endocrinology

*Reference range may vary between labs, but normal TSH is considered 0.4-4 mIU/L

**Symptoms of hypothyroidism, elevated TPO antibodies, history of atherosclerotic heart disease, heart failure, pregnancy

I’m pregnant - should I get my thyroid level checked?

Current guidelines do not recommend screening for hypothyroidism in pregnancy. If you are pregnant, we do not recommend you use TeleTest for testing your thyroid function as close medical follow-up is required. Additionally, TSH values in pregnancy between 2.5-4 require treatment but show up as normal lab values from the lab.

What are common symptoms of low thyroid function (i.e. hypothyroidism)?
  • Intolerant to cold weather

  • Unusual fatigue (> 3 months)

  • Mental slowness

  • Constipation

  • Dry Skin

  • Sensitivity to cold or heat

  • Hair loss

  • Unexpected weight gain

  • Heavy periods

  • Low mood and/or depression

  • Swelling around the eyes and puffiness of the face

What are common symptoms of high thyroid function (i.e. hyperthyroidism)?
  • Weight loss

  • Palpitations and irregular heartbeat

  • Diarrhea

  • Irritability

  • Tremor

  • Excessive sweating

  • Heat insensitivity (feeling hot all the time)

  • Fatigue

  • Skin thinning

  • Brittle hair

What is the difference between TSH and T4?

TSH is the most important hormone to determine if someone has an overactive or underactive thyroid gland. T4 (Thyroxine) is the key form of thyroid hormone that circulates in the bloodstream where it is carried into tissues and converted into a more metabolically active form T3.

  • When the body detects T4 levels are low, it releases TSH to raise T4 levels.

  • When T4 levels are high, the body suppresses levels of TSH to lower T4 levels.

  • Doctors rely initially on screening for hypothyroidism or hyperthyroidism by ordering a TSH level.

What are T3 and rT3 (reverse T3)?

Triiodothyronine (T3) is made in the thyroid gland and in tissues from the conversion of Thyroxine (T4). Approximately 80% of Triiodothyronine (T3) is made in the body’s tissues from the conversion of T4 to T3, with the rest coming from the thyroid gland[1]. T3 is more active in cells and plays a greater role than T4 in activating the cells’ machinery to carry out its necessary functions. rT3 is a metabolically inactive form of T3 and has little diagnostic utility.

What are current guideline recommendations for evaluating thyroid function?

We recommend ONLY ordering a screening TSH test to diagnose hypothyroidism or hyperthyroidism. If you have an abnormal TSH, we recommend a follow-up T4 level. We do not recommend ordering T4, T3, or rT3 as initial tests in accordance with current clinical practice guidelines.

Should I test for T3?

We understand that many patients are interested in obtaining their T3 levels. Current clinical practice does not require testing for T3 in initial thyroid testing, as most medical diagnoses can be made with a TSH and an add-on T4 blood test. A T3 level can be useful in the following scenario: an individual with hyperthyroidism (over-active) has a normal T4 and elevated T3. In this scenario, a T3 would be a useful test only after ordering a T4 test that is inconclusive. Because T3 levels can be normal in an individual with hypothyroidism, it has limited utility in diagnosing hypothyroidism.

Should I test for rT3?

Most physicians don’t test for rT3 because it provides limited information to guide a diagnosis or enable medical management of your thyroid gland. Measurement of rT3 is controversial and there is no current rationale for using rT3 to guide levothyroxine therapy. Rarely, the measurement of rT3 can be used in hospitalized patients along with measurement of their TSH, T4 and T3 to determine if a patient has hypothyroidism or sick euthyroid syndrome. In critically ill patients, rT3 can be high while TSH, T3, and T4 can be normal or low. There is controversy with respect to combination T3-T4 supplementation to treat the thyroid disease. TeleTest offers the option to test for rT3 but we do not use this lab value to guide medical decision-making.




[3] Physiological role and regulation of iodothyronine deiodinases: a 2011 update.




[7] Reverse T3 or perverse T3? Still puzzling after 40 years [CRISTIANE GOMES-LIMA et al]




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