Endocrine System Flashcards
List three selenoproteins involved in thyroid hormone homestasis and give their functions.
- Deiodinase-1 (D1) and deiodinase-2 (D2) covert T4 to T3.
- Deiodinase-3 (D3) coverts T4 to Reverse T3 (RT3).
What are the functions of the following hormones: 1. TRH 2. TSH 3. T3 4. T4 5. Reverse T3
- TRH: Stimulates TSH release from the anterior pituitary.
- TSH: Stimulates thyroid hormone production. Activates iodide uptake via the sodium / iodide symporter (SIS).
- T3: 4 x the ‘strength’ of T4. Increases growth, bone and CNS development, increases BMR, heart rate and activates metabolism.
- T4: AKA thyroxine. Approx. 90% of secreted thyroid hormone. Weak ‘thyroid’ activity — ‘inactive’ form.
- Reverse T3: Biologically inactive — protects tissues from excess thyroid hormones.
List four essential minerals and the role they play in thyroid hormone synthesis.
- Iodine: Along with tyrosine, used to form T4 and T3, catalysed by TPO.
- Iron: TPO is haem-dependent.
- Selenium & zinc: Are enzyme co-factors and important for thyroid receptor function.
- Copper: A cofactor of deiodinase enzymes.
Which vitamins support the synthesis and function of thyroid hormones?
Vitamins A, C, E, B2, B3, B6, B12
List four food sources rich in iodine.
- Sea vegetables
- ocean fish and shellfish (ie. cod, scallops)
- eggs
- dairy foods (due to the fortification of animal feed)
Which of the following promotes iodine disorders? a) Low iodine intake b) Excess iodine intake c) Both
c) Both
List two possible causes of excess iodine and the conditions this could lead to.
- An inhibited Wolff-Chaikoff effect (helps reject excess iodine and hormone synthesis)
- Over-consumption of iodised salt, iodine fortified milk and iodine-containing dietary supplements.
- Radiocontrast dyes / medications (e.g., amiodarone — used for heart arrythmias and contains iodine).
Excess iodine can cause hyperthyroidism, hypothyroidism and a goitre.
How much T4 and T3 is secreted by the thyroid daily?
The thyroid secretes approx. 80–100 mcg of T4 and 10mcg T3 daily.
Where is most T3 in the body obtained from?
Only 10% of circulating T3 is derived directly from thyroid secretion. The remaining 90% is obtained via conversion from T4 in peripheral tissues (liver and kidney) to active T3 or inactive reverse T3.
Explain why elevated Reverse T3 (RT3) levels can be problematic.
RT3 is biologically inactive but can bind to T3 receptors, blocking the action of T3. An increase in RT3 = decrease in T3.
↑ RT3 can present as hypothyroidism.
Give examples of four types of HPT disruptors that can interfere with the HPT axis, thyroid hormone synthesis, secretion, transport, metabolism and function.
- Pesticides: Alter hepatic enzymes, reducing T4 half life. Glyphosate lowers TSH with reduced gene expression for D2, D3 and transporters.
- PCBs (POPs) and bisphenols (e.g., BPA): Affect thyroid hormone receptors.
- Phthalates: Affect synthesis, metabolism and transport.
- Perchlorates: (e.g., nitrate fertilisers and food packaging) Block Na-I symporter, inhibiting iodide uptake. Can also cause lower levels of T3 in breast milk.
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Halogens: (disrupt thyroid functioning) Fluoride — in toothpaste, tap water and pesticides. Interferes with the sodium iodide symporter (= ↓ iodine uptake) and iodothyronine deiodinase (= ↓ T4 to T3 conversion). In a study, a widely-fluoridated area was almost twice as likely to report high hypothyroidism prevalence in comparison to a non-fluoridated area. The effect seems to be mitigated by adequate iodine status.
Chlorine — swimming pools, PCBs — ↑ TSH, ↑ thyroid antibodies.
Bromine — in pesticides, PBDEs (flame retardants, farmed fish).
What advice can be given to avoid HPT disruptors?
- Drink filtered water.
- Opt for fluoride-free toothpaste.
- Eat organic.
- Avoid farmed fish.
- Avoid processed foods / beverages.
- Limit time spent in chlorinated pools.
- Avoid plastic packaging.
- Select organic textile products.
- Use natural cleaning products.
Give an example of a medication that often exert the following effects on thyroid function: a) Decrease TSH secretion b) Alter T4 and T3 metabolism c) Reduce T4 to T3 conversion d) Reduce T4 and T3 binding e) Increase thyroglobulin
a) Dopamine, glucocorticoids, lithium.
b) Phenytoin, rifampicin.
c) Beta-blockers, amiodarone.
d) Diuretics, NSAIDs.
e) Oestrogen, tamoxifen.
What condition could result from the long-term use of hyperthyroid medication?
Hyperthyroid medications may induce hypothyroidism 10–20 years later in Grave’s disease.
How is the gut microbiome related to the thyroid?
Gut dysbiosis negatively affects thyroid function. Microbes regulate iodine uptake, degradation, and enterohepatic cycling.
Which gut markers are typically out of range in auto-immune thyroid disease (AITD)?
In AITD, low SCFA production is common, as is elevated zonulin (intestinal permeability) and elevated serum LPS, leading to chronic low-grade inflammation.
What are the optimal ranges for the following thyroid tests: 1. TSH 2. Free T4 3. Total T4 4. Free T3
- TSH: 0.4 to 2.5
- Free T4: 12.8 to 19.5 pmol/L
- Total T4: 70 to 150
- Free T3: 3.2 to 4.5 pmol/L
How might you interpret the following test results: a) High TSH; Normal T4; Normal T3 b) High TSH; Low T4; Low/Normal T3 c) Low TSH; High/normal T4; High/normal T3
a) Subclinical hypothyroidism
b) Hypothyroidism
c) Hyperthyroidism
What is the optimum level of iodine as measured in the first morning urine for adults, children and pregnant women?
Iodine goal = A urinary first morning iodine level of:
- 100–199 mcg / L in children and adults.
- 150–249 mcg / L in pregnant women.
Which level of urinary iodine is considered as severe deficiency?
< 20 mcg / L = severe deficiency.
Which at home test can be recommended to give an indication of thyroid status?
The Barnes basal body temperature test The theory is that waking axillary temperature is diagnostic for low thyroid, if < 36.5 degrees C.
How would the following SNPs impact thyroid function: - VDR - BC01 - GPX
- VDR: Vitamin D is a co-factor required for T3 to function correctly.
- BC01: Retinol is a co-factor required for T3 to function correctly.
- GPX: A SNP in the gene that codes for glutathione can increase oxidative stress which can disrupt the HPT axis.
How would you classify the following presentations of hypothyroidism: a) TSH is slightly elevated and T4 is normal. T4 to T3 conversion issues, ↑ RT3 or thyroid cell receptor resistance. b) Pathological processes are within the thyroid gland. TSH is higher due to low T4 and T3 (e.g., iodine deficiency, autoimmune, viral infections, drug induced, postpartum).
a) Subclinical
b) Primary
List 10 general signs and symptoms of hypothyroidism.
- Fatigue.
- Weight gain / inability to lose weight
- Heavy or irregular menstrual periods
- Puffy face, swollen eyelids, oedema
- Intolerance to cold, cold extremities
- Joint and muscle pain / weakness
- High cholesterol (usually LDL)
- Dry skin, elbow keratosis, brittle nails
- Hair loss / thinning hair & eyebrows
- Brain fog / concentration problems
- Depression
- Easy bruising
- Constipation
- Gas / bloating
- Headaches
- Low libido
- Fertility problems
- ↑ miscarriage risk
- Goitre
- Bradycardia
- Carpal tunnel syndrome