- Thyroid gland secretes 3 hormones:
- Thyroxine (T4)
- Triiodothyronine (T3)
- T4 (thyroxine) and T3 (triiodothyronine) are together known as the ‘thyroid hormones’
- Essential for normal maturation and metabolism of all the tissues in the body
- Goitre = enlarged thyroid gland, and may be associated with hypo-, eu- or hyperthyroidism
- Presence of autoimmune thyroid disorders should alert clinician to possible presence of other disorders (e.g. diabetes, autoimmune hypoparathyroidism, primary gonadal failure, Addison’s disease)
- More common in elderly, with combined prevalence of 5%
- Normal [T4] = 100nM -> deiodination (in liver and kidneys) -> Normal [T3] = 2nM
- Deiodination can also produce rT3, which is biologically inactive.
- Factors which affect peripheral conversion include:
a) Fasting (increased lipase -> increased free FAs -> displace thyroid hormones from binding sites
b) Systemic illness
c) Drugs (e.g. amiodarone, propylthiouracil)
[TSH does not have an effec]
- In plasma, over 99.95% of T4 is bound to proteins (reduces renal excretion):
5% transthyretin (aka prealbumin)
- TSH follows a diurnal rhythm, with a peak at around midday.
- Half-life of T4 is 6-7 days, and of T3 is 1-1.5 days (thus latter has more rapid rate of change).
- Total [T4] does not always reflect metabolic status, because of changes in binding protein concentration. In such instances, total [T4] will be above the reference interval, although free [T4] will be normal (due to accommodation by TSH). Causes include:
a) Pregnancy (increased synthesis by oestrogens)
b) Women on OCP (increased synthesis by oestrogens)
c) Genetics (increased or decreased synthesis)
d) Nephrotic syndrome
- Congenital hypothyroidism occurs with 1 in 4000 live births. If diagnosed at an early age, replacement thyroid hormone can be given and normal development can occur. Delays result in cretinism. Elevated TSH is diagnostic of disorders of the thyroid itself, i.e. primary neonatal hypothyroidism (taken on a Guthrie card). The TSH screening test does not pick up pituitary dysfunction in the newborn.
- Drugs can affect the thyroid gland in many ways:
i) Corticosteroids, dopaminergic drugs inhibit TSH secretion
ii) Lithium, iodine, carbimazole, thiouracils – inhibit T4 and T3 secretion
iii) Oestrogens, phenothiazines – increase TBG
iv) Androgens, corticosteroids – decrease TBG
v) Salicyclates, phenytoin – compete with T4 for binding by TBG
vi) β-blockers, amiodarone – inhibit conversion of T4 to T3
- Thyroid testing should be avoided, if possible, until a patient is well and recuperated