Tests During Medical Use of Liothyronine (T3)

Liothyronine (T3) acts quickly and briefly, so its monitoring has its own rules: a test taken at the “wrong” time can show a completely different picture. The editorial team explains which indicators doctors use, how to read them, and why how you feel does not replace the laboratory.

Why a special approach to T3 tests is needed

Laboratory monitoring during liothyronine (T3) therapy differs from monitoring during ordinary levothyroxine use. The reason is pharmacokinetics: liothyronine is absorbed quickly, its blood concentration peaks a few hours after the tablet and then declines. Therefore the same patient can have “high” or “normal” free T3 depending solely on when the blood was drawn.

The second particularity is that on T3 therapy the level of free T4 is usually reduced. The body receives the ready-made active hormone, the pituitary reduces TSH secretion, and the person’s own gland produces less thyroxine. For someone who does not know this pattern, low T4 may look like “hypothyroidism,” although it actually reflects the treatment scheme itself.

Finally, T3 acts on the heart, bones, lipid metabolism and liver, so the doctor assesses not only thyroid indicators but also the state of target organs. Tests are a way to make sure that the dose does not create hidden thyrotoxicosis.

In this article the editorial team explains which indicators are used in medical practice, how to interpret them, and which mistakes should be avoided. The material concerns only the medical use of liothyronine under a doctor’s supervision.

Basic thyroid indicators

The main reference point remains the thyroid-stimulating hormone (TSH). The guidelines of the American Thyroid Association (2014) use it as the principal marker of the adequacy of replacement therapy in primary hypothyroidism. Suppressed TSH against the background of T3 use indicates an excess of the hormone, even if the person feels well.

Free T3 helps to assess how high the concentrations of the active hormone become, but its interpretation requires knowing the time the tablet was taken. Free T4 on liothyronine monotherapy is not very informative: it is naturally reduced. On combined T4+T3 therapy both free hormones are assessed together.

In patients with central (secondary) hypothyroidism, when the pituitary is affected, TSH does not reflect the state of hormone provision. Then doctors rely on the levels of free hormones and the clinical picture.

Timing of the blood draw: the most common mistake

Since the T3 concentration first rises after taking the tablet and then falls, the test result strongly depends on the interval between the tablet and the blood draw. Blood taken 2–4 hours after intake will show a level close to the peak; blood taken before the next dose — the minimum.

The ATA guidelines (2014) emphasize that when interpreting indicators on therapy that contains T3, the time of the last dose must be taken into account. Many endocrinologists ask the patient to have the test done in a standardized way, for example each time before the morning dose, so that results can be compared over time.

For TSH, the time of day also matters: its level is higher at night and in the morning. Therefore a morning fasting blood draw is a generally accepted standard that makes comparison of results easier.

It is also worth remembering biotin. High doses of biotin in supplements for hair and nails can distort the results of immunoassays for TSH and thyroid hormones, depending on the laboratory’s method. Laboratories and doctors usually advise stopping biotin a few days before the test.

Monitoring of target organs

Thyroid hormones affect almost all tissues, so during long-term T3 therapy the doctor assesses a broader range of indicators. This applies especially to older people, postmenopausal women and patients with cardiovascular disease.

• Heart:pulse rate, blood pressure, ECG; with complaints of palpitations — Holter monitoring.
• Bones:densitometry in groups at risk of osteoporosis; markers of bone metabolism — at the doctor’s discretion.
• Lipids:a lipid panel, since deficiency and excess of hormones change cholesterol differently.
• Glucose:thyroid hormones can increase the need for glucose-lowering medications.
• Liver:transaminases in the presence of complaints or concomitant diseases.

The systematic review by Biondi and Cooper showed that even subclinical thyrotoxicosis, that is, suppressed TSH with normal free hormones, is associated with an increased risk of atrial fibrillation and loss of bone mass. That is precisely why doctors do not consider “feeling good” a sufficient criterion for a correct dose.

An electrocardiogram is useful even before the start of therapy: it provides a baseline for comparison. If palpitations, chest pain or shortness of breath appear, the patient should seek medical help immediately rather than wait for a scheduled test.

For patients with diabetes it is important to know that a change in thyroid status can change the need for insulin or other medications. This is reflected in the official instructions for liothyronine.

Frequency of examinations and interpretation

After starting therapy or changing the dose, doctors usually order a repeat test after a few weeks — TSH responds to changes more slowly than the concentration of T3 itself. Once the dose is adjusted, monitoring is done less often, as a rule once every 6–12 months or when the condition changes.

Unscheduled monitoring is appropriate in certain situations: pregnancy or its planning, a significant change in body weight, starting medications that affect thyroid hormone metabolism, and the appearance of symptoms of excess or deficiency.

Results should be interpreted as a whole. Normal free T3 with suppressed TSH, a test taken shortly after the tablet, low T4 on T3 monotherapy — all these are typical situations in which “out-of-range figures” require an explanation from a doctor rather than self-adjustment of the dose.

We note separately: the test results of people who take T3 without a prescription often look like thyrotoxicosis with reduced T4 and thyroglobulin. A doctor to whom the patient has not reported taking the drug may mistake such a picture for a disease and order an unnecessary examination. Honesty with the doctor here is a matter of safety.

Editorial conclusions

Monitoring of liothyronine therapy relies primarily on TSH, while free T3 is informative only when the time of the blood draw is taken into account. Low free T4 on T3 monotherapy is an expected result.

Alongside thyroid indicators it is important to monitor the heart, bones, lipids and glucose, especially in older people.

Best practice is a standardized draw time, stopping biotin before the test, and full information for the doctor about all medications.

Related topics on our blog: tests during medical use of levothyroxine, particulars of liothyronine for women, overdose of levothyroxine.

References

1. Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association task force on thyroid hormone replacement. Thyroid. 2014;24(12):1670–1751.
2. Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocr Rev. 2008;29(1):76–131.
3. U.S. Food and Drug Administration. Cytomel (liothyronine sodium) tablets: prescribing information. Silver Spring (MD): FDA.
4. Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343–1421.
5. Chaker L, Bianco AC, Jonklaas J, Peeters RP. Hypothyroidism. Lancet. 2017;390(10101):1550–1562.
6. Flynn RW, Bonellie SR, Jung RT, et al. Serum thyroid-stimulating hormone concentration and morbidity from cardiovascular disease and fractures in patients on long-term thyroxine therapy. J Clin Endocrinol Metab. 2010;95(1):186–193.