Generality
Triiodothyronine (or T3 ) is one of the two main hormones produced by the thyroid; in particular, it is synthesized by the cells of the tissue that surrounds the "acini" of which this gland is constituted, starting from residual substances of thyroglobulin (Tg).
In the blood, the T3 hormone circulates linked to transport proteins (mainly thyroxine-binding globulin, TBG). A smaller proportion, called FT3, is present in free (unbound) form in the blood, and can reach peripheral tissues, where it is converted into T3.
Measuring the amount of triiodothyronine (total or free) is useful for evaluating and explaining any abnormal values of TSH and / or T4 (or thyroxine).
The thyroid is a small gland shaped like a flattened butterfly, located in the neck. The hormones produced by it basically regulate the speed with which the body uses energy.
What's this
Triiodothyronine, known more simply as T3 due to its molecular structure characterized by 3 iodine molecules, is one of the two hormones released by the follicular cells of the thyroid.
T3 levels in the blood can be monitored to assess the endocrine activity of this gland, often altered in excess ( hyperthyroidism ) or defect ( hypothyroidism ).
T3 blood levels do not depend only on the amount released by the thyroid; at the peripheral level, in fact, the other thyroid hormone - thyroxine or T4 - undergoes the activity of specific enzymes, called deiodases, which deprive it of an iodine molecule converting it to T3. From a metabolic point of view, this hormone is much more active than T4, but at the same time it is much less represented in the blood.
Thanks to the peripheral conversion of T4, approximately 85% of T3 is synthesized at the level of peripheral tissues; please note that this process depends strictly on the availability of selenium.
To regulate the exposure of cells to the effects of triiodothyronine, the body has two mechanisms available:
- The first consists in the regulation of the deiodinase enzyme, which for what has been said is expressed to a greater extent when the organism needs a greater receptivity to the thyroid hormones, and vice versa;
- The second strategy involves the plasma proteins transporting thyroid hormones: albumin, transthyretin and above all TBG (acronym of Thyroid Binding Globulin, thyroxine-binding globulin).
To acquire biological activity and regulate the metabolism in the target cells, triiodothyronine must necessarily be separated from these proteins; this is why we often prefer to dose the plasma levels of the free fraction (free T3 ), rather than the absolute ones ( total T3 ).
Importance of Thyroglobulin and Free T3
As anticipated, the relationship between the two fractions - free and total - generally depends on the needs of the organism and on the activity of the thyroid ; so in hypothyroidism, despite having a total amount of low triiodothyronine we will have - at least in theory - a total T3 ratio / free T3 shifted in favor of the free quota, while in the case of hyperthyroidism (excess of thyroid hormones), the fraction linked to TBG will be proportionally greater than the free one (keeping the triiodothyronine bound to plasma proteins the body tries to defend itself from the negative effects of its presence in excess).
Otherwise, there may be situations in which the patient appears to be hyperthyroid based on the absolute value of triiodothyronine, without however showing the typical signs and symptoms of this condition; this is the case, for example, of women on estrogen therapy, in which high levels of estrogen can increase the synthesis and binding of TBG against thyroid hormones; in the face of a reduced concentration of free T3, the body tries to compensate by increasing the synthesis of these hormones, stimulated by the pituitary hormone TSH; we will therefore have high values of total T3, high values of TBG, and normal values of free T3. The opposite situation occurs during therapies with corticosteroids or in the presence of liver diseases, factors that decrease the synthesis of TBG by the liver.
To summarize : while the total T3 values can be influenced by the amount of plasma proteins binding the thyroid hormones, the measurement of free T3, being independent of this factor, is more reliable for diagnostic purposes.
Importance of T4 and TSH values
In general, the reference test to evaluate the activity of the thyroid gland is the plasma thyroxine dosage (T4), often associated with the determination of blood TSH . In some cases, however, the doctor may need to evaluate both T3 and T4 levels; for example, there are some forms of hyperthyroidism in which, with normal levels of thyroxine, the plasma values of T3 are higher than normal.
The dosage of T3 values can for example be carried out together with that of abnormal antibodies (thyroid-stimulating immunoglobulins or IST) in the presence of the classic symptoms of hyperthyroidism, in order to diagnose Graves-Basedow's disease (the most common cause of hyperthyroidism worldwide). In this disease, TSH levels appear low because abnormal antibodies bind to receptors of the same hormone mimicking the biological stimulus action on T3 and T4 synthesis.
Another important aspect is the comparison between the values of T3 with those of TSH. We have mentioned how this hormone, secreted by the pituitary gland in the center of the skull, stimulates the thyroid to produce more hormones. In the presence of hypothyroidism, and therefore of a low level of thyroid hormones in the blood, it is logical to expect high levels of TSH in an attempt to stimulate the thyroid to produce more T3; vice versa in case of hyperthyroidism it is logical to expect low values of TSH. However, there are exceptions that escape this rule, for example pituitary adenomas that can increase or decrease the amount of secreted TSH regardless of T4 or T3 levels.
Beyond these and other rare conditions, measuring TSH plasma levels is also important for monitoring the efficacy of replacement therapy undertaken in hypothyroidism conditions. For example, high TSH values may signal an insufficient intake of thyroid hormone drugs (eg eutirox); and viceversa. For therapeutic purposes, the synthetic analogue of thyroxine is preferred, while T3 can be used for the initial and rapid treatment of hypothyroidism.
Why do you measure
The triiodothyronine test allows us to determine the levels and this is useful in evaluating thyroid function .
On the blood sample, it is possible to determine both the free form of T3 (the unbound one), and the total one (bound form + free form); since most of the T3 circulates bound to proteins, the measure of total T3 can be influenced by the concentration of proteins and their binding capacity.
The blood concentration of thyroid hormones can be increased or decreased due to:
- Excessive or deficient production by the thyroid, secondary to the dysfunction of the gland itself;
- Excessive or insufficient production of TSH due to pituitary dysfunction.
The determination of T3 is normally used as a support in defining a picture of hyperthyroidism and may be required for monitoring previously diagnosed thyroid diseases .
When is the T3 hormone examination required?
The determination of total T3 and free T3 is indicated by the doctor in the following cases:
- Detection of abnormal values of TSH or T4;
- The patient presents the symptomatology of hyperthyroidism;
- It is necessary to monitor the course and efficacy of therapy during thyroid diseases previously diagnosed.
Any associated exams
Often, T3 examination is prescribed in association with TSH and FT4 (free thyroxine).
Furthermore, the search for anti-thyroid antibodies in the diagnosis of Graves' disease, an autoimmune disease that represents the main cause of hyperthyroidism, may be required.
Normal values
Unique normality values are not available for the determination of the T3 hormone. The reference intervals may differ, in fact, in the various laboratories, since they depend on many factors, including the patient's age and sex, analytical methods and instrumentation in use.
For this reason, it is preferable to consult the ranges reported directly on the analysis report. It should also be remembered that the results must be assessed as a whole by the medical doctor, who knows the patient's medical history.
T3 High - Causes
- Hyperthyroidism;
- Basedow-Graves disease;
- Infectious or autoimmune thyroiditis in the early stages (inflammatory diseases affecting the thyroid gland);
- Toxic adenoma (Plummer's disease);
- Ovarian teratoma;
- Pituitary adenoma;
- hyperpituitarism;
- TSH secreting tumors (such as, for example, some cancers of the lung, prostate or breast).
T4 Low - Causes
- Hypothyroidism;
- Thyroiditis in advanced stages;
- Poor intake of iodine;
- Thyroidectomy or chemical destruction of the thyroid;
- Peripheral conversion deficit of triiodothyronine T3 (secondary to stress, prolonged fasting, infections and chronic diseases);
- Some drug therapies (including amiodarone, interferon and lithium);
- Accumulation diseases (amyloidosis, hemochromatosis, etc.);
- Pregnancy.
How to measure it
The determination of T3 is carried out following a simple blood sample.
Preparation
The sample is usually taken in the morning. Your doctor will suggest if you need to be fasting.
In general, plasma T3 levels are not influenced by the subject's lifestyle, so fasting or abstention from particular activities before sampling is not normally required.
It should be remembered that taking some medicines can interfere with the determination of T3; this is the case, for example, of oral contraceptives and therapies for the long-term treatment of tumors. For this reason, it is advisable to communicate to the doctor all the medications that are taken.
Interpretation of Results
| Normal thyroid values | |
| Thyroxine (T4) total (TT4) | 60 - 150 nmoles / L |
| Thyroxine (T4) free (fT4) | 10 - 25 pmoli / L |
| Total triiodothyronine (T3) (TT3) | 1.1 - 2.6 nmol / L |
| Triiodothyronine (T3) free (fT3) | 3.0 - 8.0 pmoli / L |
| Thyroid-stimulating hormone (or thyrotropin) (TSH) | 0.15 - 3.5 mU / L |
NOTE: the normal ranges may vary from laboratory to laboratory; moreover, different units of measure are sometimes used (eg mcg / dl and ng / dl) and in this case the numerical values are completely different from those listed. Values may also vary based on age and pregnancy. For all these reasons, we advise you to refer to the normal ranges indicated in the analysis certificate. | |
In general, high levels of T3 are spies of an overactive thyroid, which produces an excessive amount of thyroid hormones, configuring the clinical picture of hyperthyroidism:
- Tachycardia;
- Anxiety;
- Weight loss;
- Sensation of warmth and impatience at high temperatures;
- Difficulty in falling asleep;
- Widespread tremors;
- Feeling of weakness;
- Hypersensitivity to light;
- Nervousness;
- Brittle nails and hair.
By contrast, low levels of T3 in the blood suggest thyroid insufficiency; the gland does not produce sufficient amounts of hormones and the classic symptoms of hypothyroidism appear, in many ways opposed to the previous ones:
- Weight gain;
- Dry skin;
- Constipation;
- Intolerance at low temperatures;
- Fatigue;
- Menstrual irregularity;
- Hair loss;
- Edema.
TSH | T4 | T3 | Interpretation |
High | Normal | Normal | Moderate hypothyroidism (subclinical) |
High | Bass | Low or normal | Obvious hypothyroidism |
Norma. | Normal | Normal | Euthyroidism (healthy patient) |
Bass | Normal | Normal | Moderate hyperthyroidism (subclinical) |
Bass | High or normal | High or normal | Manifested hyperirodism |
Bass | Low or normal | Low or normal | Rare hypothyroidism pituitary (secondary) |
As shown in the table, in the presence of hypothyroidism the plasma levels of T3 may also be normal or just below the norm; this brings us back to what we said at the beginning of the article, or to the body's ability to modulate T3 synthesis at a peripheral level in case of need (as happens in the presence of hypothyroidism). In such a situation we will therefore have an increased conversion of T3 at the peripheral level, with a further reduction of the already small amounts of T4.
Influence of Drugs on T3 Values
Numerous drugs can decrease or increase total T3 levels; among the first we mention anabolic steroids, androgens, anti-thyroid drugs (propylthiouracil and methimazole), interferon alpha, interleukin 2, lithium, propanolol and phenytoin; instead, they are able to increase total T3 levels: birth control pills, chlorofibrate, estrogens and methadone. For this reason, before performing blood T3 analysis it is important to inform your doctor about any ongoing drug therapies. In general, free T3 values are not affected by these drugs.
Pregnancy and Stress
Pregnancy can be accompanied by slight increases in total T3, as it tends to increase TBG synthesis.
When the organism is sick, or subject to strong stress, the peripheral synthesis of T3 decreases; so most hospitalized and cachectic patients show low plasma T3 and T4 levels.
