THYROID PROFILE: FUNCTION OF HORMONES, IMPORTANCE, QUANTIFICATION - BIOLOGY - 2025

The thyroid profile is the set of chemical tests that evaluate the function of the thyroid gland. The thyroid profile quantifies the hormones produced by the thyroid gland, these being 3, 5, 3`-triiodothyronine and 3, 5, 3`, 5-tetraiodothyronine, better known as (T3) and (T4 or thyroxine) respectively.

On the other hand, the thyroid profile also includes the measurement of a hormone synthesized in the pituitary that regulates thyroid function, called thyroid-stimulating hormone or thyrotropin (TSH).

ELISA test for FT3 and FT4, location of the thyroid gland and histological section of the thyroid tissue. Source: James Gathany; content provider (s): CDC / Hsi Liu, Ph.D., MBA, James Gathany. / The original uploader was Arnavaz at French Wikipedia., Translated by Angelito7 / Andrea Mazza

The thyroid hormones are in charge of regulating the metabolism in general. An imbalance in its production (increase or decrease) causes pathological states in the individual. Meanwhile, TSH acts on the thyroid to stimulate the release of the hormones T3 and T4 into the circulation.

The thyroid profile is perhaps the most requested study by endocrinologists due to the high frequency of patients with metabolic disorders. In general, metabolic disorders are related to a dysfunction of the thyroid gland. Among the most common pathologies associated with an altered thyroid profile are hypothyroidism, hyperthyroidism, and goiter.

Thyroid hormone function

The hormones T3 and T4 are found in two forms in the circulation. One is bound to two carrier proteins called thyroxine-binding globulin (TBG) and thyroxine-binding pre-albumin (TBPA). TBG is the most important because it is the one with the highest affinity and binding capacity.

Most of the plasma T3 and T4 is found non-covalently bound to the aforementioned proteins and only a small portion of these are free. Free T3 and T4 hormones are those with active biological activity.

The concentration of T3 and free T4 are similar, but free T4 has a longer half-life than T3. The thyroid gland produces both T3 and T4, but in the circulation T4 can be converted to T3 thanks to enzymes called deiodases.

For its part, TSH is responsible for regulating the levels of T3 and T4. When there is a decrease in the concentration of thyroid hormones, the hypothalamus is stimulated to send a signal to the pituitary gland, through a negative feedback mechanism, to produce more TSH.

That is why, in hypothyroidism, T3 and T4 are decreased and TSH elevated. While in hyperthyroidism the opposite occurs, there is an elevated plasma concentration of T3 and T4 and TSH is decreased.

Importance of performing the thyroid profile

Thyroid diseases are one of the most common endocrine disorders. As thyroid hormones are related to the regulation of metabolism in general, thyroid dysfunction is a factor in the suffering of other pathologies, such as metabolic syndrome, heart disease or obesity, among others.

Many times, thyroid dysfunction does not produce specific signs, but rather manifests with the aforementioned pathologies, therefore, the thyroid profile should be evaluated when it is suspected that the disorder may be of thyroid origin.

For the diagnosis of primary hypo or hyperthyroidism, only the TSH value is necessary. However, if there are signs and symptoms of hypothyroidism and there is an inexplicably normal value of TSH, it is necessary to evaluate FT4, but FT3 and total T3 are not essential to make the diagnosis.

On the other hand, there may be an increase in plasma total T3 concentration related to pregnancy, use of oral contraceptives, or estrogen therapy, while the FT3 concentration remains basically unchanged.

It should be noted that free T3 values ​​are more stable and are more difficult to decrease even in hypothyroidism. However, it is increased in hyperthyroidism caused by thyroid nodules.

Sometimes it is necessary to complement the study of the thyroid profile with other tests such as: thyroglobulin (TBG), scintigraphy, thyroid peroxidase antibodies (anti-TPO), antithyroglobulin antibodies, thyroid ultrasound, fine needle aspiration (FNA) and immunoglobulin thyroid stimulant (TSI), among others.

Quantification of the thyroid profile

Laboratory techniques for testing these hormones have varied over time. In the past they were less sensitive, but today they have very advanced (ultra-sensitive) methodologies.

TSH is previously mediated by RIA (Radio Immuno Assay). Today we have the IRMA (Immunoradiometric Analysis) technique and also the chemiluminescence technique.

FT3 and total T3 are measured by RIA and IRMA, while FT4 and total T4 by chemiluminescence. Also some of these determinations are available by the ELISA technique (Enzyme immunoassay).

The tests are carried out on serum. The patient does not need any type of previous preparation.

Reference values ​​of the thyroid profile

Some organizations such as the Third National Health and Nutrition Examination Survey III have tried to establish normal values ​​for these hormones.

This has not been an easy task, so other organizations such as the National Committee for Clinical Laboratory Standards recommend that normal values ​​be established in each region, using the 2.5 and 97.5 percentiles.

However, most kits for determining total T3, free T3, total T4, free T4, and TSH handle reference numbers.

- TSH

A normal TSH value helps rule out primary hyperthyroidism or hypothyroidism. It is the main determination that must be made.

TSH: 0.39 - 6.82 µIU / L.

- Free T3 and Free T4

FT3: 1.4 - 4.2 pg / mL.

FT4: 0.80 - 2.0 ng / dL.

- T3 total and T4 total

Total T3: 60-181 ng / dl.

Total T4: 4.5 and 12.5 μg / dL.

- Pregnant

TSH

First trimester: <2.5 μIU / ml.

Second trimester: 0.1-5.5 μIU / ml.

Third trimester: 0.5-7.6 μIU / ml.

T3L and T4L

FT3: 1.8-4.2 pg / mL.

FT4: 0.76 - 2.24 ng / dL.

- Elderly

TSH: 0.39 - 7.5 µIU / L.

Thyroid profile alteration

Free T3

It is augmented by:

-Hyperthyroidism (induced by drugs such as amiodarone).

-Congenital goiter (due to dysfunction of thyroperoxidase or decreased thyroglobulin).

-Patients with multinodular goiter treated with iodine (iodine thyrotoxicosis).

-Increased TSH production by pituitary tumors.

-Tyroid hormone resistance syndrome.

The determination of FT3 is useful in hyperthyroidism when TSH is very low.

FT3 is decreased in:

FT3 is the most stable hormone, therefore it is difficult to find low values. In any case, it is observed decreased when there are very high TSH values. Interestingly, FT3, being the biologically most important thyroid hormone, is the one with the least utility in the diagnosis of hypothyroidism. It is most useful in hyperthyroidism, as is total T3.

Free T4

It is increased in primary or secondary hyperthyroidism. Also in patients using oral contraceptives. It is decreased in primary hypothyroidism.

T3 total

They are increased in pregnancy, in TSH-producing adenoma, in Refetoff syndrome or resistance to thyroid hormones. They are decreased in congenital TBG deficiency, in prolonged fasting, myocardial infarction, febrile syndrome, tumors, septicemia, among others.

T4 total

It is increased in pregnancy, in chronic hepatitis, in TSH-producing adenoma, in obesity, in myasthenia gravis, Refetoff syndrome or resistance to thyroid hormones, among other causes.

It is reduced in the diet poor in iodine, hypoalbuminemia, in celiac patients, diseases that occur with loss of protein, in panhypopituarism, among other causes.

TSH

TSH values ​​above 20 μIU / L with low FT4 occur in primary hypothyroidism. Elevated TSH levels and elevated FT4 indicate TSH hyperproduction due to pituitary defect. In the case of subclinical hypothyroidism, TSH is elevated but FT4 is normal.

On the other hand, TSH values ​​below 0.1 μIU / L and high FT4 indicate primary hyperthyroidism. In subclinical hyperthyroidism, TSH is low but T4 L is normal.

Another possibility is low TSH, with normal FT4 and normal FT3, this indicates subclinical hyperthyroidism or thyroid adenoma and in the case of low TSH with normal FT4 and high FT3, it indicates TT3-toxicosis.

Finally, low TSH with low FT3 and low FT4 possible hypopituitarism.

Pathologies

Hypothyroidism

It is a disease characterized by a dysfunction of the thyroid gland and therefore there is a decrease in the production of thyroid hormones. The signs and symptoms that suggest hypothyroidism are those related to a slowdown in metabolism.

Therefore, hypothyroidism should be suspected in patients who report weakness, fatigue, drowsiness, cold intolerance, obesity, memory loss, constipation, fragility of the scalp, menstrual disorders, among others.

It is diagnosed with the determination of the TSH hormone that is elevated.

The most common cause of hypothyroidism is Hashimoto's disease, an autoimmune disease where antibodies are produced against the thyroid gland.

Hyperthyroidism or thyrotoxicosis

The most common cause is Graves' disease. This is characterized by the production of autoimmune antibodies that stimulate specific TSH receptors, generating an overproduction of T3 and T4 levels.

This situation accelerates the metabolism and, therefore, asthenia, weight loss, tachycardia, dyspnea, heat intolerance, anxiety, sweating or nervousness are observed, among others.

There are physical signs that make the diagnosis of thyrotoxicosis such as the observation of ophthalmopathy, dermopathy and clubbing or digital hypocartism. However, they are not always present and the way to make the diagnosis is through laboratory tests.

TSH is extremely low and FT4 high. In the presence of toxic thyroid nodules, hyperthyroidism presents with low TSH, normal FT4, and high free T3.

There are situations in which there may be a physiological elevation of TSH. For example, it is common to observe an increase in TSH in the first 3 months of pregnancy, which is called gestational hyperthyroidism. High concentrations of HCG can stimulate the thyroid due to its similarity to TSH.

Furthermore, thyroglobulin is increased in the pregnant state and this causes the values ​​of total T3 and total T4 to increase and those of free T4 to decrease. To calculate the normal value of total T4 of the pregnant woman, the value of T4 T of the non-pregnant woman is multiplied by 1.5.

Likewise, in the older adult stage there is a tendency for TSH levels to rise.

Goiter

The enlargement of the thyroid gland is called a goiter. The thyroid profile in these patients is varied and depends on the type of goiter. It can occur with normal, increased, or decreased hormones. That is, the gland can be normal, hyperfunctional or hypofunctional respectively.

Goiter Source: Wikipedia.com

References

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