TSH, T4, T3: Key Thyroid Hormones and Their Tests

The thyroid gland: conductor of the hormonal orchestra

The thyroid gland is a small but vital organ located in the neck area. It plays a central role in regulating the body's metabolism, growth, and development by producing thyroid hormones: thyroxine (T4) and triiodothyronine (T3). These hormones influence almost all cells and organ systems, from the cardiovascular to the nervous system. Their production is under the strict control of the pituitary gland, which secretes thyroid-stimulating hormone (TSH).

TSH is the primary regulator of thyroid function. When the level of thyroid hormones in the blood decreases, the pituitary gland increases TSH production, stimulating the thyroid gland to work more actively. Conversely, with an excess of T4 and T3, the TSH level decreases. This is why TSH is often the first and most sensitive indicator for assessing thyroid function.

Thyroid hormone tests: what do TSH, T4, and T3 show?

To fully assess the state of the thyroid gland, doctors usually prescribe a set of tests, including:

• Thyroid-stimulating hormone (TSH): As mentioned, this is a key indicator reflecting the pituitary gland's activity in relation to the thyroid. Its level allows for the detection of both hypothyroidism (elevated TSH) and hyperthyroidism (lowered TSH).
• Free thyroxine (free T4 or FT4): This is the active, non-protein-bound fraction of T4 that directly affects metabolism. The free T4 level is a direct indicator of the amount of hormone available to tissues.
• Free triiodothyronine (free T3 or FT3): Like free T4, this is the active fraction of T3. T3 is considered a more biologically active hormone than T4 and is largely formed from T4 in peripheral tissues.

Although TSH is the primary screening marker, measuring it alone does not always provide a complete picture. For example, patients after thyroid removal (athyreosis) receiving levothyroxine (LT4) replacement therapy may have normal TSH levels but low free T3 levels, which can lead to symptoms of hypothyroidism [2]. This underscores the importance of assessing free T3 and free T4 to determine the peripheral action of thyroid hormones [2].

The ratio of free T4 to free T3 (FT4/FT3) can also be informative. It is used to assess sensitivity to thyroid hormones, as well as in some cases to differentiate between benign and malignant thyroid nodules [4, 7].

When standard tests are not enough: nuances of diagnosis

In some clinical situations, standard tests may not fully reflect the patient's condition, requiring a deeper approach:

• Athyreosis and replacement therapy: In patients without a thyroid gland taking levothyroxine, normalization of TSH does not always mean optimal well-being. Studies show that a significant portion of such patients (up to 18%) may have low free T3 levels despite normal TSH and free T4, which is associated with hypothyroid symptoms [2]. In such cases, mild TSH suppression may be required to achieve normal free T3 levels and an euthyroid metabolic state [3].
• Hashimoto's thyroiditis and gland volume: When managing patients with Hashimoto's thyroiditis (HT) receiving levothyroxine monotherapy, thyroid volume (determined by ultrasound) can influence the treatment strategy and interpretation of hormone levels [3]. HT can manifest as either atrophy or diffuse enlargement of the thyroid gland [3].
• Autoimmune thyroid diseases (AITD): Some forms of AITD, such as Graves' disease or Hashimoto's thyroiditis, can have unique subtypes. For example, measuring serum IgG4 levels has been proposed to identify AITD characterized by fluctuations between hyperthyroidism and hypothyroidism [1]. Early diagnosis of IgG4-related thyroid diseases (IgG4-RTD), which can include HT, Riedel's thyroiditis, and Graves' disease, is critically important because it is a treatable condition, and delays in diagnosis can lead to unnecessary surgeries [6].

The thyroid gland and other diseases: non-obvious connections

Thyroid function and its pathologies can be linked to a range of other conditions:

• Thyroid nodules: Nodular formations in the thyroid gland are a common phenomenon. Studies show that patients with Klinefelter syndrome have an increased prevalence of thyroid nodular pathology (31% vs. 13% in the control group) [5]. To differentiate between benign and malignant thyroid nodules, the FT4/FT3 ratio combined with thyroglobulin (Tg) and parathyroid hormone (PTH) levels can be a valuable diagnostic tool. Malignant nodules often demonstrate higher FT4/FT3, Tg, PTH, and TSH values [4].
• Atrial fibrillation (AF): Even in people with normal thyroid function (euthyroid), impaired sensitivity to thyroid hormones is associated with the progression of atrial fibrillation. This is assessed using indices such as PTFQI, TT4RI, and the FT3/FT4 ratio [7].
• Breast cancer: There is a correlation between breast cancer and autoimmune thyroid diseases [8]. Although T3, T4, and free T4 levels in patients with malignant breast tumors may be within the normal range, certain changes in the concentrations of these hormones are observed compared to healthy individuals [8].

Modern approaches to treatment and monitoring

Modern management of patients with thyroid diseases is becoming increasingly personalized and comprehensive. This includes:

• Extended assessment: It is not enough to rely solely on TSH. It is important to consider free T3 and free T4 levels, especially in patients on replacement therapy, to ensure optimal peripheral hormone action and avoid symptoms of hypothyroidism [2].
• Individual approach: Treatment strategies, for example, in Hashimoto's thyroiditis, can be adapted based on thyroid volume, which allows for more precise adjustment of levothyroxine dosages [3].
• Early diagnosis: In cases of suspected IgG4-related thyroid disease, early diagnosis is crucial for prescribing medical treatment and preventing unnecessary surgical interventions [6].
• Comprehensive nodule diagnosis: In the presence of thyroid nodules, the combination of the FT4/FT3 ratio with thyroglobulin and parathyroid hormone levels can significantly improve the accuracy of differential diagnosis between benign and malignant formations [4].

Understanding the complex interaction of thyroid hormones and their influence on the entire body allows doctors to offer more effective and targeted diagnostic and treatment strategies, improving the quality of life for patients.

Sources

1. Investigation of serum IgG4 levels in the clinical metamorphosis of autoimmune thyroid diseases. (Journal of clinical & translational endocrinology, 2017) https://pubmed.ncbi.nlm.nih.gov/29067257/
2. Serum free triiodothyronine as an additional marker of thyroid hormone status in athyreotic patients receiving levothyroxine. (The Journal of clinical endocrinology and metabolism, 2026) https://pubmed.ncbi.nlm.nih.gov/42306861/
3. Thyroid hormone measurement indicators in patients with Hashimoto's thyroiditis on levothyroxine monotherapy: management based on thyroid tissue volume. (Thyroid research, 2026) https://pubmed.ncbi.nlm.nih.gov/42169022/
4. FT4/FT3 ratio combined with thyroglobulin and parathyroid hormone for the differentiation of benign and malignant thyroid nodules: a retrospective study based on histopathological diagnosis. (Frontiers in medicine, 2026) https://pubmed.ncbi.nlm.nih.gov/42755732/
5. Increased prevalence of thyroid nodular pathology in patients with Klinefelter syndrome. (Endocrine, 2023) https://pubmed.ncbi.nlm.nih.gov/37148417/
6. Immunoglobulin G4-related thyroid disease: single-center experience and literature review. (Endocrinology and metabolism (Seoul, Korea), 2022) https://pubmed.ncbi.nlm.nih.gov/35504602/
7. Impaired sensitivity to thyroid hormones is associated with atrial fibrillation in euthyroid individuals. (Frontiers in cardiovascular medicine, 2026) https://pubmed.ncbi.nlm.nih.gov/41953668/
8. Correlational study of the association between breast cancer and autoimmune thyroid diseases. (Alternative therapies in health and medicine, 2024) https://pubmed.ncbi.nlm.nih.gov/38940788/

The information provided in this article is for reference purposes only and cannot replace professional medical advice. Always consult a qualified specialist for the diagnosis and treatment of diseases.