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Describe the feedback regulatory mechanism for controlling the blood level of T3 and T4 hormones.
- Low blood levels →
- Hypothalamus (TRH or TRF – thyrotropin releasing factor) →
- Anterior pituitary (TSH) →
- Thyroid gland (T3 and T4) →
- Negative feedback to Hypothalamus (through T3)
Describe the biosynthesis, secretion, and metabolism of T3 and T4 hormones.
- Thyroglobulin – made in thyroid when stimulated by TSH. It contains lots of tyrosine. Add iodine to tyrosines.
- Part of another tyrosine is attached to the first through an ester bond.
- Each gets more iodine, either one (T3 – 3 iodines) or two (T4 – 4 iodines)
- Protease cleaves off amino acids (previously tyrosine) which is now T3 and T4.
- Released into blood, bound by carrier proteins (thyrobinding globulin) but some is free and active.
- T3 – Tri-iodo-thyronine is the most active but are lower blood levels. Half-life is 1 day.
- T4 – Thyroxine is most abundant, but may be turned to T3 in tissues—be the pro-hormone of T3. Half life is 8 days. Less % free than T3 but far more in the blood.
Distinguish between Primary and Secondary thyroid diseases, and explain what is meant by Tertiary thyroid disease.
- Primary diseases originate from the thyroid (such as a tumor of the thyroid)
- i. Hyperthyroidism
- 1. FT-4 ↑ (also FT-3, TT4, TT3)
- 2. TSH ↓ to low-normal (feedback loop is trying to reduce T3 and T4 production)
- ii. Hypothyroidism
- 1. FT-4 ↓ (also FT-3, TT4, TT3)
- 2. TSH ↑ (trying to stimulate thyroid hormone production without success)
- Secondary diseases originate from the Anterior Pituitary (such as a tumor in the pituitary)
- i. Hyperthyroidism
- 1. FT-4 ↑ (also FT-3, TT4, TT3)
- 2. TSH ↑ (pituitary overstimulating thyroid)
- ii. Hypothyroidism
- 1. FT-4 ↓ (also FT-3, TT4, TT3)
- 2. TSH ↓ (Pituitary understimulating thyroid)
- Tertiary diseases originate from hypothalamus, but are very rare.
- i. Hyperthyroidism or Hypothyroidism
- 1. Same as secondary. A test would have to rule out pituitary through something like a T3 suppression or TRH stimulation test. If pituitary responds correctly, the problem is the hypothalamus.
Identify the three transport proteins for thyroid hormones in the blood.
- Thyroid Binding Globulin – carries both T3 and T4.
- Albumin (called thyroid binding albumin or TBA) – secondary carrier for T3 and T4.
- Pre-Albumin (TBPA) (Called Transthyretin after complexing with retinal binding protein.) – carries T4 only.
Identify the most abundant blood thyroid hormone and the most potent thyroid hormone, and explain the reason for the increased potency.
- Most abundant = T4
- Most potent = T3
- Increased potency may be due to the fact that a higher percentage of T3 is free in the blood and the rest is bound. Free -
- i. 0.4% of T3
- ii. 0.04% of T4
Explain the principles involved in the following thyroid function tests, and describe the rationale for performing each test:
- Total T4 and T3
- Free T4 and T3
- Free Thyroxine Index
- Thyroid Stimulating Hormone
- Total T4 (TT4) and T3 (TT3)
- i. Total T3 has greater sensitivity than Free T3 since there is so little Free T3.
- Free T4 (FT4) and T3 (FT3)
- i. Free is the active stuff.
- ii. Free T3 changes before Free T4 does, so some think it is better to analyze T3 levels.
- T3-Uptake (Resin T3 Uptake, RT3U)
- i. Binding saturation – normally they are 1/3 saturated, carry 1/3 of their capacity. This is the same for all carrier proteins. If lower, than hypothyroid, if higher, than hyperthyroid.
- 1. If under or over-producing carrier proteins, the values won’t correctly indicate T3 levels.
- ii. Add T3I-125 (with radioactive I) to serum, causing force-saturation of carrier proteins. IgG antibodies immobilized in tube, which binds leftover radioactive T3. Leftover radioactive T3 indicates how many “spots could be filled on the bus.”
- Normal uptake: 27 – 37%
- Lower: Hypothyroid
- Higher: Hyperthyroid
- FTI (Free Thyroxine Index, FT4I)
- i. A calculation used to predict a patient’s actual thyroid status using T3-uptake and TT-4 values (or FT4 which changes the index number). Useful when T3-Uptake doesn’t correlate well with TT-4.
- • TT4 = High-normal
- • T3-uptake = High
- • TT4 = Low-normal
- • T3-uptake = Low
- ii. Reference Range: 1.1 – 4.2 (for 100%)
- 1. Within the range = Euthyroid
- 2. Lower = hypothyroidism
- 3. Higher = hyperthyroidism
- Thyroid Stimulating Hormone (TSH or Thyrotropin) by RIA.
- i. Distinguishes between primary and non-primary (secondary and tertiary) disease.
- ii. Part of the recommended annual screen, using highly sensitive TSH assay. A Free T4 is the other part, especially for hyperthyroid screening.
- iii. Been a number of generations on this test, growing more sensitive and using different methods.
- 1. Competitive RIA.
- 2. More sensitive - Sequential RIA, EMIT, ELIZA.
- 3. Highly sensitive – chemoluminescence, MEIA. Accurately detects low and high levels and offers info about severity.
- iv. Required specimen: non-hemolyzed serum. Freeze if not performing assay. Avoid repeated freeze-thaw cycles. You loose activity.
Under what conditions will patients demonstrate conflicting T4 and T3-Uptake test results and why:
- Oral contraceptive therapy or pregnancy.
- i. These cause the synthesis of TBG (thyroid binding globulin) increasing “the number of bus seats available.”
- 1. May run into a situation when T4 levels says you’re low-normal and T3 uptake says you’re hypothyroid.
- Long standing regimen of drugs:
- i. Salicylate or Acetaminophen therapy.
- ii. Antibiotic or other forms of drug therapy.
- iii. Coumadin or Heparin Therapy.
- - These drugs will be bound by TBG as well as Albumin, taking “spots on the bus” so that there aren’t any empty seats for thyroid hormone.
- - Get a T3 uptake test that is falsely hyperthyroid.
For the following thyroid diseases, predict the relative expected blood values for TSH, T4, T3- Uptake, and FTI:
- Cretinism and Myxedema
- Secondary Hypothyroidism
- Tertiary Hypothyroidism
- Hashimoto’s Disease
- Simple Goiter
- Grave’s Disease
- Toxic Goiter due to benign tumor
- Secondary Hyperthyroidism
- Cretinism and Myxedema - Primary Hypothyroid Conditions. TSH ↑. Everything else ↓.
- i. Cretinism: neonatal hypothyroidism resulting in mental retardation, voice changes (cat-like cry) pointy ears, and dwarfism. Screened for immediately after birth. Synthroid T4 therapy prevents the disease if present.
- ii. Myxedema: idiopathic adult hypothyroidism. Results in coarsening of the skin, puffiness of the skin of face, hair loss, skin legions, run-down, narcolepsis. Hereditary. It is the most common hypothyroid disease, and fully treatable with T4 therapy.
(Pituitary failure). Relatively rare. TSH ↓. Everything else ↓.
(Hypothalamus failure) Very rare. TSH ↓. Everything else ↓. Stimulation test to determine whether it is pituitary or hypothalamus.
- Hashimoto’s Disease (Chronic Thyroiditis)
- i. Autoimmune disease that slowly destroys the thyroid gland. Initially euthyroid pattern that becomes a hypothyroid pattern.
- ii. Thyroglobulin is the initial initiator of the immune response. Gobs leak out into the blood and represents a foreign protein to the immune system since levels are usually so low. As thyroid cells are damaged, more antigens are exposed and more antibodies are produced…
- iii. Quantitate Anti-Tg (Thyroglobulin) Antibodies in the blood. Can also quantitate the other antibodies to other parts of the thyroid to keep track of progression of treatment.
- iv. Early stages: everything normal except Anti-Tg elevating.
- v. Later stages: TSH High. T4, T3, FTI, T3-uptake ↓. Anti-Tg, Anti-TPO, TMAB ↑.
- Simple (Endemic) Goiter
- i. Caused by low levels of iodine, and has nearly disappeared in US where iodine is added to salt, but still exists in the world where seafood and iodinized salt is unavailable.
- ii. Overstimulation of thyroid gland causes hypertrophy; however, T3 and T4 are still low because the iodine is not available to make these hormones. TSH ↑. T4, T3, FTI, T3-uptake ↓.
- iii. Primary hypothyroid disease with goiter.
- Grave’s Disease (Exophthalmic Goiter) – characterized by bug eyes from fat deposits behind eyes pushing the eyes out.
- i. Most common form of hyperthyroidism.
- ii. TSH-receptor antibodies stimulate thyroid the way TSH would, causing over production of T3 and T4. TSH ↓. T4, T3, TFI, T3 Uptake ↑.
- iii. Without treatment, exopthamic (eye) condition will cause eyes to rupture, resulting in blindness. They also have a goiter.
- iv. Treatment with anti-thyroidal drugs (similar to iodine, used to make nonfuctional T3 and T4), plasmapheresis, or radiation therapy to destroy thyroid gland (with radioactive I) and then administer synthetic hormone.
- Toxic Goiter due to benign tumor (Thyrotoxicosis)
- i. Caused by one or more benign tumors of the thyroid which overproduce hormones (nodular goiter – nonsymmetrical). Usually does not cause exophthalmic condition.
- ii. Primary hyperthyroid condition. TSH ↓. T4, T3, FTI, T3 Uptake ↑.
- Secondary Hyperthyroidism (Pituitary microadenoma) – fairly uncommon.
- i. Caused by a microadenoma (pituitary tumor), which usually produces more hormones than just TSH. Tumor may be visible on CATscan. TSH ↑. T4, T3, FTI, T3 Uptake ↑.
– Thyroglobulin elevates in the blood in Grave’s Disease, Hashimoto’s, Thyroiditis, Benign and Melignant Thyroid tumors, as well as from physical trauma to the thyroid. It is most helpful as a disease marker. Blood levels relate to progression and severity of disease.
Describe the National Academy of Clinical Biochemistry (NACB), AACC, and the Endocrine Society’s recommendations for the preliminary screening for Hypothyroidism and Hyperthyroidism.
- Screen TSH annually.
- If suspecting hyperthyroidism, add Free T4 screen.
- If either of these is abnormal, other tests are added.
Describe the clinical utility of measuring Parathyroid Hormone (PTH), as well as Total and Ionized Calcium and Inorganic Phosphorous for the diagnosis of Hyperparathyroidism or Hypoparathyroidism.
- Parathyroid Hormone (PTH) increases Ca2+ levels, and is turned on and off by negative feedback of ionized calcium levels of blood. Can raise calcium through three mechanisms:
- i. Increased reabsorption of filtered calcium in urine filtrate.
- ii. Increase bone breakdown to liberate the calcium-rich bone salts (hydroxyapatite).
- iii. Increased production of protein needed to absorb calcium from GI.
- iv. Calcitonin is PTH’s antagonist, made by the thyroid gland.
- v. Disorders involving PTH are common, but not so with calcitonin.
- Hyperparathyroidism = overproduction of PTH, so ionized blood calcium levels are ↑.
- i. Causes: benign tumor (most common) or hyperplasia
- ii. Symptoms: muscle pain, spasms, and weakness due to high ionized calcium, arthritis (crystals from high calcium), kidney stones (calcium phosphate).
- iii. Total Ca2+ is increased, but the important elevated fraction is ionized calcium.
- iv. Bone osteolysis releases both Ca2+ and Phosphate. Ca2+ is reabsorbed from urine filtrate but phosphate excretion is even higher.
- • Total calcium ↑. Inorganic Phosphorous ↓ or low normal. Ionized calcium ↑. Serum PTH ↑.
- Hypoparathyroidism – most likely patient previously had a parathyroidectomy or thyroidectomy
- • PTH ↓. Calcium ↓. Inorganic Phosphate ↑.
Identify the major PTH-derived peptides (PTH fragments) in the blood, and explain which fragments are preferred for commercial RIA methodologies:
- PTH fragments:
- • Amino-Terminal Fragment (34 AA) – half-life of only 15 – 30 minutes. PTH has same.
- • Carboxy-Terminal Fragment (several) – half-life of 2 – 4 hours.
- • Mid-Molecule Fragment (several) – half-life of 2 – 4 hours.
There are methods specific to each fragment.
Methodologies testing for Carboxy-terminal fragments and mid-molecule fragments are preferred since there are more of them and they have a longer half-life means greater chance of detection and greater specificity.