Thyroid Hormones: Synthesis and Regulation

• Thyroid and parathyroid function can be assessed via laboratory tests, external radionuclide scans, and ultrasonography.

Thyroid Gland

• Located in the anterior neck, resembling a small bow tie across the trachea.
• The adult thyroid weighs about 20g.
• Composed of left and right lobes connected by a small isthmus.
• Produces thyroid hormones (T4, T3) and calcitonin.
• Thyroid hormones require iodine, an essential trace element for production.
• Hormones are stored in thyroid colloid, an extracellular site containing proteinaceous material.
• Thyroglobulin is the major protein containing thyroid hormones.
• Thyroid follicular cells surround thyroglobulin and synthesize thyroid hormones.
• C cells (parafollicular cells) synthesize calcitonin, playing a role in calcium and phosphate regulation.

Thyroid Hormone Production

• Thyroid-stimulating hormone (TSH) binds to receptors, signaling follicular cells to ingest colloid droplets via endocytosis.
• Droplets are digested by intracellular lysosomes into T4, T3, and other products.
• T4 and T3 are secreted by thyroid cells into circulation.
• Free T4 and T3 diffuse into target cells, where 80% of T4 is converted to T3 by 5'-deiodinase.
• T3 is three to eight times more active than T4 and is considered the active form. T4 is considered the "pre"-hormone.
• Four steps are involved in thyroid hormone synthesis:
  • Inorganic iodide is trapped from circulating blood.
  • Iodide is oxidized to iodine.
  • Iodine is added to tyrosine to produce monoiodotyrosine and diiodotyrosine (organification).
  • One monoiodotyrosine is coupled with one diiodotyrosine to yield T3.
  • Two diiodotyrosines are coupled to yield T4 (coupling).
• Secreted T4 and T3 circulate in the bloodstream almost entirely bound to proteins.
• Free T3 is biologically active and mediates the effects of thyroid hormone on peripheral tissues.
• Thyroxine-binding globulin (TBG), synthesized in the liver, is the major binding protein.
• TBG binds one molecule of T4 or T3.
• Thyroid-releasing hormone (TRH) is released by the brain, stimulating TSH release from the pituitary gland.
• TSH stimulates iodine uptake and the release of T3 and T4 from the thyroid gland.
• High serum levels of free T3 and T4 inhibit TSH release, while decreased levels induce TSH release.

Screening

• High-risk individuals for thyroid screening:
  • Neonates
  • Pregnant and postpartum females
  • Elderly individuals
  • Those with a family history of autoimmune or thyroid disease

Diagnosis and Indicators

• Diagnosis of hyperthyroidism and hypothyroidism includes TSH and free thyroxine (fT4).
• FT4 is the most reliable indicator of thyroid status in sick individuals.
• Disorders are characterized by the site of the defect:
  • Primary: Disease originates in the thyroid gland.
  • Secondary: Disease originates in the pituitary gland.
  • Tertiary: Disease originates in the hypothalamus.

Laboratory Tests

• Serum of plasma samples are used
• Samples should be free from hemolysis and lipemia
• Newborn screening uses whole blood collected by heel puncture within 48 hours

Assessing Thyroid Function

• Tests available to assess thyroid function:
  • TSH
  • Free thyroxine (fT4)
  • Total thyroxine (TT4)
  • Free triiodothyronine (fT3)
  • Total triiodothyronine (TT3)
  • Thyroglobulin
  • Thyroid antibodies
  • Thyroxine-binding globulin (TBG)
• Immunoassays are used for the determination of hormone levels, with the plasma concentrations of most hormones being extremely low.

RIA (Radioimmunoassay)

• Part 1: A specific antibody is mixed with a labeled hormone, creating a radioactive antigen-antibody complex.
• Part 2: Patient serum is added to the mix. The unlabeled hormone competes with the labeled hormone for binding to the antibody.
• Part 3: Free hormone and antibody-hormone complex are separated. The radioactivity measures the free, unbound hormone. A large amount of hormone in the patient's serum leads to high specific radioactivity of the free hormone.
• TSH is a screening test for thyroid status.
• Increased TSH joined by increased fT4 is secondary (pituitary), causing thyrotoxicosis and hyperactivity of the thyroid.
• Increased TSH joined by low fT4 is primary hypothyroidism.
• Low TSH and low fT4 indicate secondary hypothyroidism.
• Low TSH and high fT4 indicate primary hyperthyroidism.
• Serum total T4 and T3 levels are usually measured by radioimmunoassay (RIA), chemiluminometric assay, or similar immunometric technique.
• Alterations in thyroid hormone-binding proteins can lead to abnormal total T4 and T3 levels without reflecting true clinical thyroid dysfunction. Assays to measure free T4 and T3 (biologically active forms) were developed.
• Current method includes measurement of fT4 using immunometric assays.
• Hypothyroidism: increased TSH is the earliest abnormality, followed by decreased serum levels of T4 and T3.
• Hyperthyroidism: the initial evaluation reveals elevated thyroid hormone serum levels and decreased serum TSH.

Thyroglobulin

• A protein synthesized and secreted by thyroid follicular cells.
• Only made by thyroid cells; its presence in the blood indicates thyroid tissue is present.
• Used as a marker for monitoring thyroid cancer after treatment.
• In patients treated successfully with surgery and radioactive iodine, thyroglobulin levels should be undetectable. If not, it may indicate remaining or returning cancer.
• Measured using lab techniques like RIA, ELISA, IRMA, and ICMA.

Thyroid Antibodies

• Many thyroid gland diseases are related to autoimmune processes.
• In autoimmune thyroid disease, antibodies are directed at thyroid tissue with variable responses.
• TRAB (TSH receptor antibodies): Cause hyperthyroidism/Graves disease. The antibodies stimulate the TSH receptor, leading to thyroid growth and hormone production. Diagnosed with tests that detect TSHR stimulating antibodies.
• TPO Antibodies: Thyroid peroxidase (TPO) is an enzyme that helps make thyroid hormones. It adds iodine to thyroglobulin and links the pieces together to form T3 and T4. Autoantibodies directed against TPO in thyroid microsomes are detected in autoimmune thyroiditis. Autoantibodies can block TPO's function, damage thyroid cells, and lead to thyroid disorders like Hashimoto's thyroiditis or hypothyroidism.

Antibody Levels

• Ranges:

  • 100 IU/ml = Positive

  • 60-100 IU/ml = Equivocal
  • <60 IU/ml = Negative
• High thyroid antibodies indicate hypothyroidism.
• Antithyroglobulin antibody (TgAb) was the first antibody discovered against a thyroid protein, thyroglobulin.

Thyroxine Binding Globulin (TBG)

• In the blood, most T3 and T4 are bound to proteins including albumin and TBG.
• Acts as a T4 reservoir; T4 is active when released from plasma proteins.
• Thyroid hormones travel in blood mostly bound to proteins.
• Increased binding proteins can change total T3 and T4 levels, but free T3, T4, and TSH remain normal, meaning thyroid function is unchanged.
• Pregnancy and oral contraceptives raise thyroid-binding protein concentrations.
• Cirrhosis and nephrotic syndrome cause lower concentrations of thyroid-binding proteins.

Parathyroid Glands

• Affect the kidney in three ways:
  • Increase the reabsorption of renal tubular calcium.
  • Increase phosphate excretion.
  • Enhance 1α-hydroxylation of 25-hydroxy vitamin D.
• PTH, vitamin D, and calcitonin bring about:
  • The mobilization of calcium and phosphate from the skeletal system.
  • Increased calcium uptake in the intestine.
  • Phosphate excretion via the kidneys.
• Secretion of PTH is inhibited by high calcium concentrations and promoted by low calcium concentrations.
• Immunoassay methods are used for in vitro quantitative determination of intact parathyroid hormone in human serum and plasma. Ratios of intact hormone to peptide fragments may vary between individuals and patients with hyperparathyroidism or chronic renal failure. Concentration of metabolically inactive PTH fragments increases in renal failure.

Imagistic Investigations

Thyroid Ultrasound

• High-resolution ultrasonography (US) is sensitive for examining the thyroid gland and abnormalities.
• Ultrasound scanning is non-invasive, available, less expensive, and does not use ionizing radiation.
• Real-time ultrasound imaging guides diagnostic and therapeutic interventional procedures.

Ultrasound Limits

• Ultrasound examines structure but cannot assess function, blood tests, or radioactive uptake test.
• It helps evaluate thyroid nodules (size, shape, blood flow, calcifications) but cannot confirm or rule out cancer. A biopsy may be needed.
• Indications for Ultrasound:
  • Confirm presence of thyroid nodule when physical exam is equivocal
  • Characterize a thyroid nodule
  • Identify suspect malignant thyroid masses
  • Differentiate between thyroid nodules and other cervical masses
  • Evaluate diffuse changes in thyroid parenchyma
  • Detect post-operative residual or recurrent tumor
  • Screen high-risk patients for thyroid malignancy
  • Guide diagnostic FNA cytology/biopsy and therapeutic interventional procedures
• Normal thyroid glands have 2 lobes and a bridging isthmus.
• Thyroid size shape volume varies = Age & Sex.

Volumes

• Female: 10-15 ml
• Male: 12-18 ml

Elastography

• Used for virtual palpation of the thyroid nodule, providing information on tissue stiffness noninvasively
• Malignant tumors often have abnormally firm stroma due to collagen and myofibroblasts (desmoplastic transformation).
• To differentiate benign from malignant thyroid nodules. By elastography elasticity assessments can be obtained.

Visual Scoring

• Visual scoring colors within and around the nodules can be assessed, 4-5-scale scoring systems. The likelihood of malignancy Increases with an increase in the strain ratio, where 1 is most elastic, and 5 the firmest .

Thyroid Scan and Uptake - Nuclear Medicine Investigation.

• Non invasive and Painless IV
• Radioactive materials (radiopharmaceuticals or radiotracers) are used during the test
• Molecules linked to or labelled with radioactive material detected on PET scan
• A camera provides information about thyroids size/shape/position/function

Procedure

• Supplement and medications involving iodine containing solution should be avoided 1-2 weeks prior. Patient should not have iodine contrast radiological studies performed 4-8 weeks prior.

Iodines Commonly Preformed

• Tc-99m pertechnetate, thyroid scintigraphy, lodine-I3I, thyroid uptake or lodine 123 uptake

Indications

• Used for localization of parathyroid adenoma or hyperplasia in patients with biochemically proven hyperparathyroidism.
• Tc-99m sestamibi is currently the preferred radiotracer for parathyroid imaging.

Parathyroide Radionuclide Scan. Combined Thyroid and Parathyroid Imaging

• I-Technetium-99m pertechnetate or I-I23 for thyroid phase
• II-Technetium-99m MIBI or Tc-99m tetrofosmin or thallium-201 chloride
• Examination is based on differential washout of Tc-99m MIBI from thyroid tissue vs parathyroid tissue. Washout rate from parathyroid tissue (adenoma) is slow vs normal thyroid tissue

Radiopharmceuticals

• Distributions of the two tracers can be visibly compared.
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