Thyroid Flashcards
Hypothalamus-Pituitary-Thyroid (H-P-T) axis
Thyrotropin-releasing hormone (TRH) stimulates thyroid-stimulating hormone (TSH) release; TSH stimulates thyroid hormone release — T4 (around 90%) and T3
Iodothyronine deiodinases
Iodothyronine deiodinases are selenopr regulating thyroid hormone homeostasis.
‒ Deiodinase-1 (D1) and deiodinase-2 (D2) covert T4 to T3.
‒ Deiodinase-3 (D3) coverts T4 to Reverse T3 (RT3)
TRH
Stimulates TSH release from the anterior pituitary
TSH
Stimulates thyroid hormone production.
Activates iodide uptake via the sodium / iodide symporter (SIS).
Triiodothyronine (T3)
4 x the ‘strength’ of T4. Increases growth, bone and CNS development, increases BMR, heart rate and activates metabolism.
Tetraiodothyronine (T4)
AKA thyroxine. Approx. 90% of secreted thyroid hormone. Weak ‘thyroid’ activity — ‘inactive’ form
Reverse T3 (RT3)
Biologically inactive — protects tissues from excess thyroid hormones
SIS
SIS = a transmembrane glycoprotein needed for the active transport of iodine into thyroid follicular cells
SIS
SIS = a transmembrane glycoprotein needed for the active transport of iodine into thyroid follicular cells
Thyroid hormone synthesis: Nutritional status
- Tyrosine and iodine: Thyroid peroxidase (TPO) catalyses iodination of tyrosine residues in thyroglobulin to form T4 and T3 (a pro-oxidant process). iodination = a reaction in which iodine is introduced
- Iron: TPO is haem-dependent (assess iron status).
- Selenium and zinc: Enzyme co-factors and receptor function.
- Vitamin D: Immune modulation in autoimmune thyroid disorders (AITD) and VDR polymorphism (shown to predispose to AITD).
- Vitamins A, C, E, B2, B3, B6, B12: Support synthesis and function.
- Copper: A cofactor of deiodinase enzymes.
Iodine rich foods
- Sea vegetables; ocean fish and shellfish such as cod and scallops; eggs and dairy foods (due to the fortification of animal feed) with small amounts in plants depending on the soil
Iodine rich foods
- Sea vegetables; ocean fish and shellfish such as cod and scallops; eggs and dairy foods (due to the fortification of animal feed) with small amounts in plants depending on the soil
Iodine deficiency — causes
- Dietary deficiency.
- Increased risk – low/no dairy/fish, pregnant, vegans.
- High goitrogen intake (e.g., soya, millet peanuts, pine nuts and raw brassicas) — goitrins, thiocyanates and nitriles in foods ↓ iodine uptake and have anti-TPO activity
Excess iodine — causes:
- The Wolff-Chaikoff effect helps reject excess iodine and hormone synthesis. The effect is inhibited in certain individuals leading to induced subclinical or clinical hypothyroidism.
- Consumption of over-iodised salt, animal milk rich in iodine (fortified), iodine-containing dietary supplements.
- Radiocontrast dyes; medications (e.g., amiodarone — used for heart arrythmias and contains iodine)
T3 and T4:
- The thyroid secretes approx.
80–100 mcg of T4 and 10mcg T3 daily.
- Only 10% of circulating T3 is derived directly from thyroid secretion.
- Remaining 90% is obtained via ‘peripheral conversion’ from T4.
- T4 is converted in peripheral tissues (liver and kidney) to active T3 or inactive reverse T3. The hormones are metabolised by deiodination, sulphation and glucuronidation. Consider detoxification protocols.
- T4 is highly bound (99.98%), 0.02% circulates freely.
- T3 is slightly less protein bound (99.8%), 0.2% as free fraction
T4 is converted to T3 (D1 and D2) or RT3 (D3 enzyme). * RT3 — biologically inactive but can bind
to T3 receptors, blocking the action of T3. Increase in RT3 = decrease in T3!
- ↑ T4 → RT3 increases in:
– Chronic / critical illness — a normal response to ↓ metabolism states — called Low T3 Syndrome.
– High stress (cortisol); zinc, selenium or iron deficiency; liver dysfunction and fasting / significant caloric restriction, advancing age (liver / kidney function), myocardial infarction.
- ↑ RT3 can present as hypothyroidism
HPT disruptors
HPT disruptors: Interfere with 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. (Patrick, 2009; Oliveira et
Can also cause lower levels of T3 in breast milk.
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)
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
Medications — often exert effects on thyroid function
Medications — often exert effects on thyroid function:
- Decrease TSH secretion: Dopamine, glucocorticoids, lithium.
- Alter T4 and T3 metabolism: Phenytoin, rifampicin.
- Reduce T4 to T3: Beta-blockers, amiodarone.
- Reduce T4 and T3 binding: Diuretics, NSAIDs.
- Increase thyroglobulin: Oestrogen, tamoxifen.
- Hyperthyroid medications may induce hypothyroidism 10–20 years later in Grave’s disease.
Gut microbiome and thyroid
- Gut dysbiosis negatively affects thyroid function.
- Microbes regulate iodine uptake, degradation, and enterohepatic cycling.
- In AITD, low SCFA (short chain fatty acid) production is common, as is elevated zonulin (intestinal permeability) and elevated serum LPS → chronic low-grade inflammation
Conventional medicine diagnostic
Thyroid Investigations interpretation
Comprehensive thyroid function testing
Measuring iodine
Measuring iodine — urine iodine test:
- Iodine goal = A urinary first morning iodine level of:
✓ 100–199 mcg / L in children and adults. What do you make of
✓ 150–249 mcg / L in pregnant women. you this result? address How this? would
- < 100 mcg / L in children and non-pregnant adults = iodine insufficiency.
- < 20 mcg / L = severe deficiency
Barnes basal body temperature test
Barnes basal body temperature test:
- The theory is that waking axillary temperature is diagnostic for low thyroid, if < 36.5º C.
- Useful indicator, but not diagnostic. Take for 7 days upon awakening, before any activity and calculate average.
- Issues: Circadian changes, illness, menstrual changes, sedentary lifestyle, light exposure, alcohol intake can give false readings. Does not correlate with core body temp
Physical Thyroid examination
Physical examination:
- Thyroid gland enlargement, dry skin, nail beading, thinning eyebrows, hair loss, low blood pressure, bradycardia (< 60 BPM)
