Basic Testing Information

Thyroid Level Testing for Thyroid Cancer Patients

This is a basic outline of the thyroid tests most frequently asked about and are needed to properly monitor thyroid hormone replacement (THR) levels for thyroid cancer patients. 

TSH – Thyroid Stimulating Hormone: is a hormone produced in the pituitary gland, a small organ located below the brain, and stimulates a normal thyroid to release the hormones thyroxine (T4) and triiodothyronine (T3) into the blood.

  •  TSH responds to the levels of T4 and T3 circulating in the blood; as T4 and/or T3 levels rise, TSH levels go down.
  • Initial TSH suppression of 0.1– 0.5 mU/L is recommended [ATA].  The rationale to keeping TSH low (suppressed) is to discourage any remnant thyroid tissue in the body from being stimulated to  prevent recurrence of the thyroid cancer.  
  • As we no longer have a thyroid after thyroidectomy, TSH is a poor measurement of metabolic response to THR therapy.

T4 (Total T4, thyroxine): the total amount of thyroxine circulating in your blood. Most of it is bound to protein (storage mode). It has small variations after ingestion but, because it has a long half life, can take a week or more to show changes from increasing or decreasing levels. T4 contains 4 iodine ions, when one of these is removed in the body it becomes the “active” thyroid hormone T3. T4 is often the primary medication prescribed for THR.

Free T4 (fT4):  the amount of circulating T4 which is “unbound” and available to be converted to T3. Most of this conversion takes place in the liver and kidneys, with some taking place peripherally—in muscles, the heart and brain—by enzymes called deiodinases.  

T3 (Total T3, triiodothyronine):  the total amount of T3 which has been converted from T4 by removal on one iodine ion, thus becoming T3. Total T3 includes both free T3 and reverse T3 (which can form when T4 is excessive or in response to inflammatory illness).

Free T3 (fT3, triiodothyronine free): the amount of “active” T3 circulating in your blood. This is the actual hormone that works in your cells to provide energy and may be the best correlation with how your body functions and feels. If you take Cytomel (synthetic T3) or NDT (desiccated thyroid containing both T4 and T3), T3 these levels will fluctuate during the day – higher shortly after you take your meds, lower 6-8 hours later – due to the shorter half-life. A stable baseline level is usually established in a few days and represents free T3 from both ingested T3 and T3 which has been converted from any T4 you also take. 

Reverse T3 (rT3): the amount of inactive T3 which has been converted from T4 and will be eliminated from the body.

Thyroid test levels can be affected by factors other than your meds including diet, illness, exercise and other hormone levels. Labs are usually drawn as a “trough” level – in the morning before taking any meds or at least six to eight hours following the last dose.  If taking supplements containing biotin, they should be stopped at least 72 hours before labs are drawn.

You can have too much T4  and too little free T3 levels and causing a mix of both “hyper” and “hypo” symptoms at the same time, which is why TSH, Free T4 and Free T3 should measured at the same time. Any GP/PCP/NP/PA can order these — and in all but five states you can order them yourself.

Annual follow up for thyroid cancer patients includes checking Thyroglobulin and an ultrasound examination. If there are any suspicious results they are repeated more frequently.
Thyroglobulin (Tg) is produced only by thyroid cells so can be used as a tumor marker to monitor for reccurrence after a complete thyroidectomy. If you have positive Tg antibodies (TgAb), a test should be used which avoids interference. — either mass spectrometry or RIA.
Ultrasound examination is used to look for changes in cervical lymph nodes and any new nodules arising from remnant thyroid tissue.  A complete ultrasound “mapping” of lymph nodes in the neck provides a baseline in case there are any changes in the future.
Medullary thyroid cancer (MTC) patients also have the unique cancer markers Calcitonin and CEA checked.

“Normal” laboratory ranges are developed based on “normal” (i.e., with intact thyroids) people. Lab ranges and the units used can vary from lab to lab and also may be slightly different in different regions of the country. 

“Normal” Ranges from four different Arizona labs:

TSH (0.45 – 4.5 mU/L) (0.35 – 4.00 uIU/mL)
Total T4 (4.5 – 12.0 ug/dL)
Free T4 (0.7 – 1.5 ng/dL) (0.8 – 1.7 ng/dL)
Total T3 (71 – 180 ng/dL)
Free T3 (2.0 – 4.8 pg/mL) (2.0 – 4.4 pg/mL)
Reverse T3 (9.2 – 24.1 ng/dL)
Thyroglobulin (1.5 – 38.5 ng/mL) (after TT, goal is zero)
Tg Antibodies (0.0 – 0.9 IU/mL)

TSH “suppression” for thyroid cancer is usually targeted below 0.1-0.2 uIU/mL. The American Thyroid Association guidelines recommend:

(A) For high-risk thyroid cancer patients, initial TSH suppression to below 0.1 mU/L is recommended.
(B) For intermediate-risk thyroid cancer patients, initial TSH suppression to 0.1– 0.5 mU/L is recommended.
(C) For low-risk patients who have undergone remnant ablation and have undetectable serum Tg levels, TSH may be maintained at the lower end of the reference range (0.5– 2mU/L) while continuing surveillance for recurrence.

It’s been suggested that after total thyroidectomy (TT) many of us feel best with Free T4 mid-range or lower and Free T3 in the mid to upper end of the range. While an individual’s values may fall within a “normal” range, the actual levels where a post-TT patient functions well should be individualized based on objectively evaluating clinical presentation rather than relying solely on lab values. Any one taking T3, either synthetic or from DTE, should have Free T3 tracked and may have TSH levels below the low end of the normal TSH range. A low TSH does not indicate over medication if free T4 and T3 remain within range and the patient is not experiencing symptoms.

More in depth information about thyroid tests can be found at:
Lab Tests Online
Biomedee’s Blog