Thyroid Function Tests

Thyroid Function Tests (TFT)

• TFTs help develop student knowledge about the thyroid gland
• TFTs help develop student knowledge about the clinical chemical aspects and pathological conditions of the thyroid gland.

Learning Objectives

• Learning objectives include understanding the anatomy and functions of the thyroid gland
• Learning objectives include learning how thyroid functions can be impaired
• Learning objectives include understanding factors affecting thyroid binding proteins
• Learning objectives include knowing how to assess thyroid function disorders
• Learning objectives include how to interpret thyroid hormone chemical laboratory test results.

Thyroid Hormones

• Thyroid hormones are vital for normal growth, development, and metabolism
• Thyroid hormone production is tightly regulated through the hypothalamic-pituitary-thyroid axis

Synthesis and Metabolism

• Thyroxine (T4), small amounts of tri-iodothyronine (T3) and reverse T3 (rT3) are synthesized in the thyroid gland
• Synthesis includes trapping iodide from plasma using a sodium-iodide symporter in the thyroid
• Synthesis includes oxidation of iodide to iodine by thyroid peroxidase
• Synthesis includes incorporation of iodine into tyrosyl residues on thyroglobulin in the colloid of the thyroid follicle, forming mono-iodotyrosine (MIT) and di-iodotyrosine (DIT)
• Production of T4 and T3 occurs via the coupling of iodotyrosyl residues in the thyroglobulin molecule
• T4 and T3 split off from thyroglobulin following its reabsorption from the colloid
• T4 and T3 are then released into the circulation

T3 and T4 Hormones

• Thyroxine is a pro-hormone produced exclusively by the thyroid
• T3 is the biologically active hormone
• Approximately 85% of plasma T3 is formed from outer-ring (5') mono-deiodination of T4 in the liver, kidneys, and muscle
• Thyroxine undergoes inner-ring (5) mono-deiodination in non-thyroidal tissues
• This process causes the production of metabolically inactive rT3

Plasma T3 Hormone

• Plasma T3 is a poor indicator of thyroid hormone secretion, because the conversion of T4 to T3 can be reduced
• Systemic illness, prolonged fasting, and drugs like beta-blockers (e.g., propranolol or amiodarone) can reduce T4
• Plasma T3 concentration is influenced by non-thyroidal factors, so measurement is rarely indicated, except in suspected thyrotoxicosis
• Plasma total T3 (tT3) is almost always raised in hyperthyroidism, usually proportionally greater than tT4, making it a more sensitive test
• Plasma total T3 may be normal in hypothyroidism due to T3 production in the thyroid and increased peripheral formation from T4

Comparison of T3 & T4 Hormones

• T4 Hormone

  • Free Fraction: 0.03%
  • Relative Potency: Mainly a prohormone
  • Half-life (T1/2): 6 to 7 days
  • Source: 100% thyroid

• T3 Hormone

  • Free Fraction: 0.3%
  • Relative Potency: Nearly 3-4 times more potent
  • Half-life (T1/2): 1 to 2 days
  • Source: 20% thyroid, 80% peripheral deiodination of T4 (largely in the liver, kidneys, and muscle, catalyzed by selenium-containing iodothyronine deiodinases)

Regulation of Thyroid Function

• TSH is the most important regulator of thyroid homeostasis
• TSH production is controlled by a stimulatory effect of TRH and mediated by negative feedback from circulating free T3 (FT3) and free T4 (FT4)
• The pituitary acts as a 'thyroid-stat' to maintain thyroid hormone production, determined by the hypothalamus
• Dopamine, somatostatin, and glucocorticoids inhibit TSH release
• Interleukins may modify TSH release in non-thyroidal illness (NTI)

Thyroid Hormones Transportation

• Thyroid hormones are transported in plasma almost entirely bound, reversibly, to plasma proteins
• Proteins that carry thyroid hormones include thyroxine-binding globulin (TBG), transthyretin (thyroxine-binding pre-albumin), and albumin
• More than 99.8% of thyroid hormones circulate bound to these proteins
• Changes in the concentration of these proteins affect total T4 and T3 concentrations
• Approximately 0.05% of plasma T4 and 0.2% of plasma T3 are free (unbound to protein), independent of changes in protein concentration and affinity
• Only free fractions can cross the cell membrane and affect intracellular metabolism

TBG Hormone

• A decrease in plasma TBG concentration reduces bound T4 concentrations without altering free T4 (fT4) concentration
• This decrease may occur due to severe illness, temporary loss of low-molecular-weight proteins (e.g., nephrotic syndrome), or androgen/danazol treatment, and rare inherited TBG deficiency
• An increase in plasma TBG concentration increases bound T4 without changing free T4 (fT4) concentrations
• Increase might happen during high estrogen concentration (pregnancy and oestrogen therapy), in newborns, or in inherited TBG excess (rare)

T4 Hormone

• T4 is metabolised to T3 inside cells, which then binds to specific nuclear receptors to activate T3-responsive genes
• These gene products modify cell functions, including basal metabolic rate and the metabolism of lipids, carbohydrates, and proteins
• High concentrations of thyroid hormone increase the basal metabolic rate and stimulate the breakdown of protein and lipids
• Low concentrations of thyroid hormone result in low metabolic rate, weight gain, and poor physical and mental development in children

Diagnosing Thyroid Status

• Measure plasma TSH and free T4 (fT4) or total T4, and free T3 (fT3) or total T3 to assess thyroid hormone secretion
• High TSH levels (primary hypothyroidism) and low TSH levels (secondary or pituitary hypothyroidism) are indicators
• High plasma T4 and T3 concentrations suppress TSH release from the pituitary, resulting in very low or undetectable plasma TSH concentrations (hyperthyroidism)
• Plasma TSH assays are used as first-line tests for the assessment of thyroid function

Thyrotrophin-Releasing Hormone (TRH) Test

• TRH test is used to confirm the diagnosis of secondary hypothyroidism
• TRH test is rarely used to diagnose hyperthyroidism
• TRH test is used in the differential diagnosis of thyroid resistance syndrome or TSH-secreting pituitary tumours (TSHomas)

Indications for TFTs

• Clinical suspicion of thyroid disease
• Newborn heel prick test for TSH to detect neonatal hypothyroidism early
• Individuals with drug exposure (Amiodarone, Lithium)
• Those with a history of neck irradiation
• Autoimmune diseases: as in T1DM

Disorders of the Thyroid Gland

• Hypothyroidism is due to deficient thyroid hormone secretion
• Hyperthyroidism is due to excessive thyroid hormone secretion

Primary Thyroid Disorders

• Defined by either a decrease or increase in thyroid hormones with reciprocal changes in TSH
• Considered subclinical if TSH is abnormal while thyroid hormones are still within normal ranges
• Subclinical thyroid disorders are common

Thyrotoxicosis

• Thyrotoxicosis is a clinical state marked by inappropriately high levels of circulating thyroid hormones (T3 and/or T4)
• It is often confused with a form of hyperthyroidism caused by excessive endogenous thyroid hormone
• Subclinical hyperthyroidism has a persistent abnormality in TSH with normal thyroid hormone levels.
• Patients with multinodular goitre or with exophthalmic Graves’ disease have ‘subclinical hyperthyroidism’, that is, TSH below 0.01 mU/L
• Before diagnosis, causes of a low TSH should be excluded, including NTI, pregnancy and drugs that suppress TSH (dopaminergic drugs, high dose glucocorticoids)
• The tests should be repeated 1–2 months later and if the abnormalities persist the patient should be referred to an endocrinologist

Subclinical Hypothyroidism (SCH)

• Subclinical primary hypothyroidism presents with normal FT4 and elevated TSH; many cases are transient
• Tests should be repeated at 3 months to exclude a transient rise in TSH

Non-Thyroidal Illness (NTI)

• NTI is the most common biochemical abnormality of endocrine function among medical inpatients
• NTI implies an illness manifesting abnormal TFTs in absence of intrinsic thyroid disease
• Euthyroid sick syndrome has TFT abnormalities with no clinical features of thyroid dysfunction and no benefit from treatment

Auto Antibodies in Thyroid Disease

• Most thyroid disorders are autoimmune
• TPOAb (Thyroid Peroxidase Ab) is the highest titer antibody present in ~90% of patients with Hashimoto’s thyroiditis
• TRAb (Thyroid Receptor Ab/ TSHR Ab) TSI are IgG antibodies directed against TSH receptors in the thyroid
• TRAbs implicates in the pathogenesis of Graves’ disease

Laboratory findings In Hypothyroidism

• Primary hypothyroidism: Elevated TSH, low T4, T3
• Secondary hypothyroidism: Decreased TSH, low T4, T3
• Tertiary hypothyroidism: Decreased TRH, decreased TSH, low T4, T3
• Subclinical primary hypothyroidism: Elevated TSH, normal TH

Laboratory findings In Hyperthyroidism

• Primary hyperthyroidism: Elevated thyroid hormones, free and total suppressed TSH levels
• T4-Toxicosis: Elevated T4 only
• T3-Toxicosis: Elevated T3 only
• Pituitary tumour-producing TSH (secondary hyperthyroidism): Elevated TSH, elevated T4, elevated T3
• Subclinical hyperthyroidism: Decreased TSH, normal free T4 and free T3

Thyroglobulin Assays

• Differentiates between Graves` disease (elevated) and factitious thyrotoxicosis (decreased)
• Determines the amount of thyroid tissue after a thyroidectomy
• Monitors the recurrence of common thyroid cancers (follicular cell-derived tumours)

Calcitonin Assay

• Detects and monitors the recurrence of medullary thyroid cancer