THYROID FUNCTION TEST

The  thyroid gland is an endocrine gland in the neck consisting of two connected lobes.
The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus.
The thyroid is located at the front of the neck, below the Adam's apple. Microscopically, the functional unit of the thyroid gland is the spherical thyroid follicle, lined with follicular cells (thyrocytes), and occasional parafollicular cells that surround a lumen containing colloid.
THYROID HORMONES:
The thyroid gland secretes three hormones: the two thyroid hormones – triiodothyronine (T3) and thyroxine (T4) – and a peptide hormone, calcitonin.
The thyroid hormones influence the metabolic rate and protein synthesis, and in children, growth and development. 
Calcitonin plays a role in calcium homeostasis.
 Secretion of the two thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is secreted from the anterior pituitary gland. TSH is regulated by thyrotropin-releasing hormone (TRH), which is produced by the hypothalamus.

1. THYROID FUNCTION TEST PRESENTED BY: Shaista sumayya P harmD Sultan ul Uloom college of pharmacy, Hyderabad.

2. THYROID GLAND The  thyroid gland is an endocrine gland in the neck consisting of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thyroid is located at the front of the neck, below the Adam's apple. Microscopically, the functional unit of the thyroid gland is the spherical thyroid follicle, lined with follicular cells ( thyrocytes ), and occasional  parafollicular cells that surround a lumen containing colloid. THYROID HORMONES: The thyroid gland secretes three hormones: the two thyroid hormones –  triiodothyronine (T3) and  thyroxine (T4) – and a peptide hormone, calcitonin. The thyroid hormones influence the metabolic rate and protein synthesis, and in children, growth and development.  Calcitonin plays a role in calcium homeostasis.  Secretion of the two thyroid hormones is regulated by thyroid-stimulating hormone (TSH), which is secreted from the anterior pituitary gland. TSH is regulated by  thyrotropin -releasing hormone (TRH), which is produced by the hypothalamus.

3. NORMAL PHYSIOLOGY OF THYROID GLAND

4. Thyroid function test ( tft ) Thyroid function tests are usually done to find out whether the thyroid gland is working properly. This is mainly to diagnose an underactive thyroid gland (hypothyroidism) and an overactive thyroid gland (hyperthyroidism ). The principal laboratory tests recommended in the initial evaluation of thyroid disorders are the TSH and the FT4 levels . Positive thyroid antibodies indicate an autoimmune thyroid etiology. Adjuncts to the previous tests include the total T3 (TT3), free T3 (FT3) or FT3 index (FT3I), RAIU and scan, TRAb , ultrasound, and FNA biopsy

5. Tft classification Measurement of Circulating Hormone Levels Free Thyroxine ( FT4) Free Thyroxine Index ( FT4I) Total Thyroxine ( TT4) Total Triiodothyronine ( TT3) Free Triiodothyronine ( FT3) Free Triiodothyronine Index ( FT3I) Tests of Thyroid Gland Function  radioactive iodine uptake  (RAIU) Scan Test of Hypothalamic-Pituitary-Thyroid Axis thyroid-stimulating hormone ( TSH)

6. Tests of Autoimmunity antithyroglobulin antibody( ATgA ) Thyroid peroxidase(TPO) thyrotropin receptor antibody( TRAb ) Miscellaneous Thyroglobulin

7. Measurement of Circulating Hormone Levels Tests Measures Normal Comments FT4 Direct measurement of free thyroxine 0.7–1.9 ng / dL (9–24 pmol /L) Most accurate determination of FT4 levels; might be higher than normal in patients on thyroxine replacement FT4I Calculated free thyroxine index T4 uptake method: 6.5–12.5 TT4 × RT3U method: 1.3–4.2 Estimates direct FT4 measurement; compensates for alterations in TBG TT4 Total free and bound T4 5–12 mcg/ dL (64–154 mmol /L) Specific and sensitive test if no alterations in TBG TT3 Total free and bound T3 70–132 ng / dL (1.1–2 nmol /L) Useful in detecting early, relapsing, and T3 toxicosis . Not useful in evaluation of hypothyroidism FT3 Direct measurement of free T3 0.2–0.42 ng / dL (3.5–6.5 pmol /L) Most accurate determination of FT4 levels; might be lower than normal in patients on thyroxine replacement FT3I Calculated free T3 index 17.5–46 Estimates direct FT3 measurement; compensates for alterations in TBG

8. Tests of Thyroid Gland Function Tests Measures Normals Comments RAIU Gland’s use of iodine after trace dose of either 123 I or 131 I 5%–35% Useful in hyperthyroidism to determine RAI dose in Graves’; does not provide information regarding hormone synthesis Scan Gland size, shape, and tissue activity after 123 I or 99m Tc — Useful in nodular disease to detect “cold” or “hot” areas

9. Test of Hypothalamic-Pituitary-Thyroid Axis Tests Measures Normal Comments ATgA Antibodies to thyroglobulin <1 IU/mL Present in autoimmune thyroid disease; undetectable during remission TPO Thyroperoxidase antibodies <1 IU/mL More sensitive of the two antibodies; titers detectable even after remission TRAb Thyroid receptor stimulating antibody <125% Confirms Graves’ disease; detects risk of neonatal Graves’

10. Test of Hypothalamic-Pituitary-Thyroid Axis Tests Measures Normal Comments TSH Pituitary TSH level 0.5–5 μ U/mL Most sensitive index for hyperthyroidism, hypothyroidism, and replacement therapy MISCELLANEOUS Tests Measures Normal Comments Thyroglobulin Colloid protein of normal thyroid gland <56 ng /mL Marker for recurrent thyroid cancer or metastases in thyroidectomized patients

11. Indications of TFT Diagnosing thyroid disorder in symptomatic person Screening newborns for hypothyroidism Monitoring thyroid replacement in hypothyroidism patients Diagnosis and monitoring female infertility patients Screening adults for thyroid disorders

12. Abnormalities of thyroid

13. hypothyroidism Causes: PRIMARY(high TSH) Autoimmune: Hashimoto’s, Atrophic Iatrogenic: subtotal/total thyroidectomy, external neck irradiation Drugs: lithium, antithyroid drugs, p-amino salicylic acid Congenital hypothyroidism: absent/ectopic thyroid gland, dyshormonogenesis , TSHR mutation Iodine deficiency Infiltrative disorders SECONDARY(low TSH) Hypopitutarism : tumors, surgery, irradiation, infiltrative disorders, Sheehan’s syndrome, trauma Isolated TSh deficiency or inactivity TERTIARY(low TSH , low TRH) Diseases of hypothalamus TRANSIENT: Silent/postpartum thyroiditis, subacute thyroiditis

14. Clinical features of hypothyroidism

15. Evaluation of hypothyroidism

16. hyperthyroidism PRIMARY HYPERTHYROIDISM Graves disease Toxic MNG Toxic adenoma Functioning thyroid carcinoma mets Activating mutation of TSH receptor Struma ovarii Drugs: iodine excess ( jod basedow phenomenon) SECONDARY HYPERTHYROIDISM TRH syndrome Chorionic gonadotropin secreting tumors Gestational thyrotoxicosis THYROTOXICOSIS WITHOUT HYPERTHYROIDISM Subacute thyroiditis Silent thyroiditis Other causes: amiodarone , radiation, infarction of adenoma Ingestion of excess thyroid hormone

17. Clinical features of hyperthyroidism

18. Evaluation of hyperthyroidism

19. tsh First line test in thyroid function test Normal TSH level excludes thyroid dysfunction TSH used alone as a first line test will miss unsuspected cases of secondary hypothyroidism , therefore combine TSH and T4 as first line tests . Uses: Screening for euthyroidism Screening for hypothyroidism in newborns Diagnosis of primary and secondary hypothyroidism Diagnosis of clinical and subclinical hyperthyroidism Follow up of T3 and T4 replacement therapy in hypothyroidism

20. TSH increase Primary hypothyroidism Addison’s disease Anti TSH antibodies Pitutary adenoma Postoperative Thyroiditis Drugs: amiodarone , bensaarazide , clomiphene, lithium, methimazole , metochlorpramide , propylthiouracil decrease Primary hyperthyroidism Hashimoto’s thyroiditis Drugs: ASA, heparin, ketoconazole, T3, dopamine, glucocorticoids

21. Total thyroxine Includes free thyroxine and protein bound thyroxine The FT4 are the most reliable tests for the evaluation of hormone concentrations, especially when thyroid hormone binding abnormalities exist . falsely elevated levels of TT4 are common in the euthyroid pregnant woman .

22. Thyroxine increase Hyperthyroidism Pitutary TSH secreting pituitary tumor Raised TBG decrease Primary hypothyroidism Secondary/pituitary hypothyroidism Severe non thyroidal illness Decreased TBG

23. Thyroid binding globulin (TBG) N ormal levels: 13 – 39 ug /dl increase Drugs: clofibrate , estrogen, oral contraceptives, heroin, methadone Genetic Pregnancy Acute intermittent porphyria hyperproteinemia decrease Drugs: androgens, glucocorticoids, phenytoin, large doses of salicylates Malnutrition Hypoproteinemia , nephrotic syndrome Cushing’s syndrome

24. Free and total t3 Normal plasma level of T3 are very low Metabolically more active, shorter half life, faster turn over Free T3 0.5% of total The FT3 is most useful in hyperthyroidism but can be normal or low in hypothyroidism TT3 is often low in older patients and in many acute and chronic nonthyroidal illnesses because the peripheral conversion of T4 to T3 is decreased. The TT3 is particularly helpful in detecting early relapse of Graves’ disease and in confirming the diagnosis of hyperthyroidism despite normal TT4 levels . Uses: Diagnosis of T3 thyrotoxicosis Early diagnosis of hyperthyroidism

25. thyroglobulin Reflects thyroid mass, thyroid injury, TSH receptor stimulation INCREASED IN: Graves disease Thyroiditis Nodular goiter INDICATIONS: Monitoring recurrence of certain variants of thyroid ca Thyroid dysgenesis in congenital hypothyroidism Follow up of patients with thyroid malignancy

26. Thyroid autoantibodies Diagnosing autoimmune diseases Autoantibodies : Tg , thyroid microsomal Ag, TSH receptor, non Tg colloid antigen, TSH, T4 ANTI Tg ANTIBODIES POSITIVE: hashimoto’s thyroiditis grave’s disease myxedema nontoxic goitre thyroid cancer

27. Antimicrosomal antibodies Positive in grave’s disease and hashimoto’s thyroiditis More frequently positive for autoimmune diseases than Tg Ab THYROID RECEPTOR ANTIBODY Types: TBI (grave’s disease) TSIgs - grave’s disease Predicting relapse or remission in hyperthyroid Develoopment of neonatal hyperthyroidism

28. Radioiodine uptake studies - correlates with functional activity of thyroid gland - tracer dose of I 131 orally followed by measurement of amount of radioactivity over thyroid gland at 2hrs and again at 24hrs - normal radioactive uptake is 20-40% of administered dose at 24hr Increased uptake Hyperthyroidism due to grave’s disease Toxic MNG Toxic adenoma TSH secreting tumor Decreased uptake Hypothyroidism Subacute thyroiditis Large I2 doses , thyroid hormone Factitious hyperthyroidism

29. Trh stimulation test Uses: confirms diagnosis of secondary hypothyroidism Evaluation of suspected hypothalamic disease Procedure: TRH injected iv (200ug) followed by measurement of serum TSH at 20 and 60 min Interpretation: Primary hypothyroidism - exaggerated and prolonged response Secondary hypothyroidism - blunted response Tertiary hypothyroidism - response is delayed

30. T3 suppression test Use: Differentiates borderline high normal form primary hyperthyroidism TSH STIMULATION TEST Use: Differentiates primary from secondary hypothyroidism

31. Thyroid scan Uses: Functional classification of nodules – warm, hot, cold Provides information regarding size, shape, position of gland Identification and localisation of functioning thyroid tissue in ectopic or metastatic sites Helps in differentiating various causes of thyrotoxicosis

32. Indications of thyroid scan Thyroid nodule Diffuse or multinodular goiter Clinical hypo or hyperthyroidism Rule out ectopic thyroid tissue Subacute thyroiditis, early phase Contraindications: Pregnancy lactation

33. NORMAL COLD NODULE HOT NODULE GRAVES DISEASE TOXIC MNG

34. Measurements of Free and Total Serum Hormone Levels - Free Thyroxine , Free Thyroxine Index, Free Triiodothyronine , and Free Triiodothyronine Index The FT4 and FT3 are the most reliable tests for the evaluation of hormone concentrations, especially when thyroid hormone binding abnormalities exist. The FT3 is most useful in hyperthyroidism but can be normal or low in hypothyroidism. If a direct measure of the free hormone levels are not available, the estimated free hormone indices (FT4I, FT3I) can provide comparable information. However , these indices do not correct for changes observed in patients with “ euthyroid sick” nonthyroidal illnesses whose TBG binding affinity is altered. In these circumstances, the FT4 and FT3 are preferable.

35. Total Thyroxine and Total Triiodothyronine The total thyroxine (TT4) and total triiodothyronine (TT3) measure both free and bound (total) serum T4 and T3. Because the bound fraction is the major fraction measured, situations that change the hormone’s affinity for TBG or the TBG level will influence the results. For example, falsely elevated levels of TT4 and TT3 are common in the euthyroid pregnant woman . In addition, the TT3 is often low in older patients and in many acute and chronic nonthyroidal illnesses because the peripheral conversion of T4 to T3 is decreased. Therefore , careful interpretation of these tests is necessary in situations that alter thyroid hormone binding, TBG levels, or T4 to T3 conversion. The TT3 is particularly helpful in detecting early relapse of Graves’ disease and in confirming the diagnosis of hyperthyroidism despite normal TT4 levels . The TT3 is not a good indicator of hypothyroidism because TT3 can be normal.

36. Thyroid autoantibodies These are markers of autoimmune thyroid disease . Antithyroid microsomal antibodies have been identified as antithyroid peroxidase (ANTITPO) antibodies . Antimicrosomal antibodies are much more sensitive than antithyroglobulin antibodies and are present in around 45–80% of Graves’ disease and 80–95% of Hashimoto’s disease/atrophic thyroiditis. Increasingly , labs are measuring anti-TPO directly as their only antibody test. Note that anti-TSH receptor antibodies—the cause of Graves’ disease—are difficult to measure and not routinely assayed. Although they are the most reliable test for diagnosing Graves’ disease, currently their only definite indications are to determine the cause of thyroid disease in pregnancy and the post-partum period and assess the risk of neonatal thyrotoxicosis

37. Thyroid function testing An undetectable TSH level and a free T3 level are required to diagnose hyperthyroidism. In milder cases, T4 levels may be in the normal range (‘T3 toxicosis ’). Normal TSH levels with 4 T4 and T3 are seen in TSHsecreting pituitary tumours (very rare) or in patients with thyroid hormone resistance (also very rare).

38. Investigation of cause

39. Anti-TSH receptor antibody testing This test is not routinely available in most labs. Although it is positive in >90% of cases of Graves’ disease, in most cases it does not alter clinical management. Indications include distinguishing gestational thyrotoxicosis or post-partum thyroiditis from Graves’ disease, indicating the risk of neonatal thyrotoxicosis and (controversial) predicting recurrence after a course of thionamide drug therapy

40. Biochemical diagnosis 4 TSH with T4 in the normal range is referred to as subclinical hypothyroidism. 4 TSH with 5 T4 is overt hypothyroidism. 5 T4 with TSH in the normal range may be due to pituitary failure (2° hypothyroidism) and if Repeat tests in 6 weeks Now eu -/ hypothyroid Spontaneously resolving thyroiditis Hyperthyroidism confirmed (suppressed TSH, raised free T3) Short history (

41. Differential diagnosis (causes) n iodine sufficient countries, the vast majority of spontaneous hypothyroidism is due to autoimmune thyroiditis (Hashimoto’s disease if goitre present, atrophic thyroiditis if goitre absent)— antithyroid antibodies present in 80–90% of cases. Other common causes are post-thyroidectomy, post-radioiodine therapy and side effects of amiodarone or lithium. Rarer causes include treatment with cytokines (e.g. interferons , GM-CSF, interleukin-2), vast excess iodine intake (iodine drops, water purifying tablets), congenital hypothyroidism (caused by a variety of genetic defects, should be detected by neonatal screening programme), iodine deficiency (urinary iodide excretion

42. Diagnostic catches 4 TSH and 5 T4 always represents hypothyroidism. If the TSH alone is 4 and the T4 is not even slightly low, a heterophile antibody interfering in the TSH assay may be present in the patient’s serum. This is especially likely if there is no change in TSH level after thyroxine treatment but the T4 level rises (confirming compliance with tablets). For unusual patterns of thyroid function tests, see Fig. 2.14. Note that within the first 1–3 months (or longer) after treatment of hyperthyroidism, profound hypothyroidism may develop with a 5 T4 but the TSH may still be suppressed or only mildly raised due to the long period of TSH suppression prior to treatment. Raised TSH alone with disproportionate symptoms of lethargy may be seen in hypoadrenalism . If suspected treat with steroids first as thyroxine may precipitate an Addisonian crisis.

43. Transient hypothyroidism Transient or self-resolving hypothyroidism, often preceded by hyperthyroidism, is seen in viral thyroiditis, after pregnancy (post-partum thyroiditis) and in some individuals with autoimmune thyroiditis (positive antithyroid antibodies). Treatment temporarily with thyroxine is only required if the patient is very symptomatic. Thyroid function should return to normal within 6 months. Hypothyroidism may also be transient in the first 6 months after radioiodine therapy .

44. Subclinical hypothyroidism A raised TSH (< 20mU/L ) with normal T4/T3 is very common and seen in 5–10% of women and ~2% of males. It is usually due to subclinical autoimmune thyroid disease and is frequently discovered on routine testing. In randomised trials, ~20% of patients obtain psychological benefit from beginning T4 therapy, in many others it is probably truly asymptomatic. If antithyroid antibodies are detectable, the rate of progression to overt hypothyroidism is ~50% at 20 years, but higher than this with higher initial TSH levels. If the TSH alone is raised with negative antibodies (or the TSH is normal with raised antibodies alone), overt hypothyroidism develops in 25% at 20 years. A reasonable approach is a trial of thyroxine

45. Hypothyroidism and pregnancy Overt hypothyroidism is associated with poor obstetric outcomes subclinical hypothyroidism is associated with a slight reduction in the baby’s IQ and should be treated . Many authorities advocate screening for hypothyroidism in all antenatal patients as early as possible in pregnancy. Patients on T4 need to increase their dose by 50g from the first trimester of pregnancy . Maternal thyroxine can compensate for fetal thyroid failure in utero but congenital hypothyroidism must be detected at birth (screening test) to avoid mental retardation. Where the mother and fetus are both hypothyroid—most commonly due to iodine deficiency—mental retardation can develop in utero ( cretinisim ). Mothers with positive antithyroid antibodies and/or subclinical hypothyroidism have a 50% chance of developing (transient) post-partum thyroiditis.

46. Thyroid Function Test Results in Different Thyroid Conditions Total T4 Free T4 Total T3 T3 Resin Uptake Free Thyroxine Index TSH Normal 4.5–10.9 mcg/ dL 0.8–2.7 ng / dL 60–181 ng / dL 22% to 34% 1.0–4.3 units 0.5–4.7 mIU /L Hyperthyroid ↑↑ ↑↑ ↑↑↑ ↑ ↑↑↑ ↓↓ Hypothyroid ↓↓ ↓↓ ↓ ↓↓ ↓↓↓ ↑↑ Increased TBG ↑ Normal ↑ ↓ Normal Normal

47. Patterns of thyroid function tests

48. Cases to avoid tft In very ill patients, especially in intensive care, a pattern of ‘sick euthyroidism ’ is often seen, with low TSH levels, low free T3 levels and sometimes low free T4 levels. Accurate interpretation of true thyroid status is impossible. A raised free T3 level in a very ill patient suggests significant hyperthyroidism and a very raised TSH (>20mU/L) with undetectable free T4 levels suggests profound hypothyroidism. Other changes should be interpreted with extreme caution and the tests repeated after recovery

49. Factors That Can Significantly Alter Thyroid Function Tests in Euthyroid Patients

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