Thyroid Gland

• Thyroid gland


  Also known as the butterfly shaped gland
• Thyroid gland

  • Consists of two lobes (one on either side of the trachea) located in the lower part of the neck just below the voice box
  • The lobes are connected by a narrow band called the isthmus
  • By 11 weeks of gestation, the gland begins to produce measurable amount of hormone
• Follicle


  The fundamental structural unit of the thyroid gland
• Types of cells in the thyroid gland

  • Follicular cells
  • Parafollicular cells or C-cells
• Parafollicular cells or C-cells


  • Situated in clusters along the interfollicular surface or spaces
  • They produce the polypeptide calcitonin, for calcium regulation
• Triiodothyronine (T3)

  • It has the most active thyroid hormonal activity
  • Almost 75-80% is produced from the tissue deiodination of T4
  • The principal application of this hormone is in diagnosing T3 thyrotoxicosis
  • A better indicator of recovery from hyperthyroidism as well as the recognition of recurrence of recurrence of hyperthyroidism - it is helpful in confirming the diagnosis of hyperthyroidism, especially in patients with no or minimally elevated T4
  • An increase in the plasma level of T3 is the first abnormality seen in cases of hyperthyroidism
• Tetraiodothyronine (T4)


  • The principal secretory product
  • It has the major fraction of organic lodine in the circulation
  • A prohormone for T3 production
  • All circulating T4 originates in the thyroid gland - it is secreted 100% in the thyroid gland
  • The amount of serum T4 is a good indicator of the thyroid secretory rate
  • Elevated thyroxine causes Inhibition of TSH secretion and vice versa
• Thyroid hormones binding proteins

  • Thyroxine-Binding Globulin (TBG)
  • Thyroxine-Binding Proalbumin (transthyretin)
  • Thyroxine-Binding Prealbumin (transthyretin)
• Thyroid hormones responsible for autoimmune disorders

  • Thyroperoxidase (TPO)
  • Thyroglobulin (Tg)
  • TSH receptor (TR)
• Screening of thyroid disorders is recommended when a person reaches 35 years old and every year thereafter
• Hyperthyroidism

  Refers to an excess of circulating thyroid hormones
• Signs and symptoms of hyperthyroidism

  • Tachycardia, tremors, weight loss, heat intolerance, emotional lability and menstrual cycle changes
• Primary hyperthyroidism

  Elevated T3 and T4, decreased TSH
• Secondary hyperthyroidism

  Increased FT4 and TSH (due to primary lesion in the pituitary gland)
• Hyperthyroidism-Thyrotoxicosis

  Applied to a group of syndromes caused by high levels of Free Thyroid hormones in the circulation
• Types of thyrotoxicosis

  • T3 thyrotoxicosis or plummer's disease: FT3 is increased but FT4 is normal with low TSH
  • T4 thyrotoxicosis: T3 is normal of low but T4 is increased with low TSH
• Primary Hypothyroidism-Hashimoto's disease

  • Chronic autoimmune thyroiditis
  • It is the most common cause of primary hypothyroidism
  • It is characterized by a thyroid replaced by a nest of lymphoid tissue-sensitized T lymphocytes/autoantibodies bind to cell membrane causing cell lysis and inflammatory reaction
  • It is associated with enlargement of the thyroid gland (goiter)
  • Lab result: high TSH and positive TPO antibody
• Hyperthyroidism-Subclinical hyperthyroidism

  Shows no dinical symptoms but TSH level is low, and TF3 and FT4 are normal
• Hyperthyroidism-Subacute granulomatous

  • Subacute nonsuppurative thyroiditis, De Quevain' thyroiditis (painful thyroiditis)
  • It is associated with neck pain, low-grade fever and swings in thyroid function test
  • The thyroidal peroxidase (TPO) antibodies are absent, ESR and thyroglobulin levels are elevated
• Hypothyroidism


  • It develops whenever insufficient amounts of thyroid hormone are available to tissues
  • It is treated with thyroid hormone replacement therapy (levothyroxine)
• Signs and symptoms of hypothyroidism

  • Bradycardia, weight gain, coarsened skin, cold intolerance and mental dullness
• Hypothyroidism-Subclinical Hypothyroidism


  Lab result. T3 and T4 are normal but TSH is slightly increased
• TRH Stimulation Test (Thyrotropin Releasing Hormone)


  • It measures the relationship between the TRH and TSH secretions
  • It is used to differentiate euthyroid and hyperthyroid patients
  • It may also be helpful in the detection of thyroid hormone resistance syndromes
  • It is used to confirm borderline cases and euthyroid Grave's disease
• TSH Test


  • The most important thyroid function test
  • The best method for detecting clinically significant thyroid disfunction
  • It is the most clinically sensitive assay for the detection of primary thyroid disorders
  • It helps in the early detection of hypothyroidism
  • It is used to differentiate primary hypothyroidism from secondary hypothyroidism
  • It is used to monitor and adjust tryroid hormone replacement therapy
  • The sensitivity of the third generation TSH assay has led to the ability to detect what is termed as subclinical disease-or a mild degree of thyroid dysfunction (due to large reciprocal change in TSH levels seen for even small changes in FT4)
• Interpretation of TSH values in Congenital Hypothyroidism

  • TSH value <10 mIU/L - no further test
  • TSH value 10-20 mIU/L - repeat test in 2-6 weeks
  • TSH value >20 mIU/L - for endocrinologic evaluation to diagnose hypothyroidism
• Radioactive lodine Uptake (RAIU)

  • Used to measure the ability of the thyroid gland to trap iodine
  • It is helpful in establishing the cause of hyperthyroidism
  • Radioactive iodine is ingested by mouth and measured after 4-6 hours and 24 hours
• Thyroglobulin (Tg) assay


  • Normally used as a postoperative marker of thyroid cancer
  • It is used in monitoring the course of metastatic or recurrence of thyroid cancer (a well differentiated tumor typically displays a 10-fold increase in Tg in response to a high TSH)
  • When measuring Tg as a tumor marker for thyroid cancer, always check simultaneous sample for thyroglobulin antibodies
  • Differentiates subecule thyroidits (increased Tg) from thyrotoxicosis factitia (decreased Tg)
• Thyroglobulin (Tg) assay results

  • Increased: untreated and metastatic differentiated thyroid cancer, nodular goiter and hyperthyroidism
  • Decreased: infants with goilorous hypothyroidism and thyrotoxicosis factifial (decreased Tg)
• Reverse T3 (rT3)

  • Rt3 is formed by removal of one iodine from the inner ring of T4
  • It is an end product of T4 metabolism; the 3rd major circulating thyroid hormone
  • It identifies patients with euthyroid sick syndrome (elevated rT3)
  • It is used to assess the borderline or conflicting laboratory results
• Free Thyroxine Index (FTI or T7)


  • It indirectly assesses the level of Free T4 in the blood
  • It is based on equilibrium relationship of bound T4 or FT4
  • It is important in correcting euthyroid individuals
  • It is elevated in hyperthyroidism and decreased in hypothyroidism
• Total T3 (TTT3), Free T3 (FT3), and Free T4 (FT4)


  • FT4 test is used to differentiate drug induced TSH elevation and hypothyroidism
  • The value of TT3 or FT3 is in confirming hyperthyroidism
  • Direct/reference method: Equilibrium dialysis (FT4)
• Fine Needle Aspirate (FNA)


  The most accurate tool in the evaluation of thyroid nodules
• Follicular Cells

  • Secretory and produce thyroxine - T4 and triiodothyronine - T3, FT3, FT4
  • Each follicle is in the shape of a sphere that surrounds a viscous substance called colloid. The major component of colloid is the Thyroglobulin, which is rich in tyrosine
• Thyroid Hormone

  • There is more T4 than T3 in serum, but T3 is more potent and significant physiologically
  • About 80% of circulating T3 is formed following monodeiodination of T4 in peripheral tissues
  • Thyroid hormones are almost completely protein bound
• Hyperthyroidism - Grave's disease


  • Diffuse toxic goiter
  • Most common cause of thyrotoxicosis
  • An autoimmune disease in which antibodies are produced to activate the TSH receptor
  • It occurs 6x more commonly in women than in man
  • Features: exophthalmos (bulging eyes) and pritibial myxedema
• Hyperthyroidism - Riedel's thyroiditis


  The thyroid turns into a woody or stony-hard mass
• Hypothyroidism - Congenital Hypothyroidism


  • Also known as cretinism
  • It is a defect in the development or function of the gland defective from birth
  • Symptoms: physical and mental development of the child are retarded
  • Screening test T4 (decreased)
  • Confirmatory test: TSH (increased)
• Primary Hypothyroidism


  • Primarily due to deficiency of elemental lodine
  • T3 and T4 are decreased while TSH is increased
  • It is also caused by destruction or ablation of the thyroid gland
  • Other causes: surgical removal of the gland, use of radioactive iodine for hyperthyroidism, radiation exposure, drugs such as lithium
• Hypothyroidism - Myxedema


  • it describes the peculiar nonpitting swelling of the skin
  • the skin becomes infiltrated by mucopolysaccharides
  • Clinical features: "puffy" face, weight gain, slow speech, eyebrows thinned, dry and yellow skin, anemia
  • myxedema coma - severe form of primary hypothyroidism
• Hypothyroidism - Secondary Hypothyroidism


  • due to pituitary destruction or pituitary adenoma
  • Lab result: T3 and T4 levels are low, TSH is also decreased