Basic thyroid function tests

Abnormal hormone production is usually due to an abnormal Primary abnormalities of thyroid-stimulating hormone (Tsss) and trophin-releasing hormone (rneai) are very rare.

Basic physiology The hypothalamus secretes rRH, a tripeptide, stimulates rhe production of TSH, a polypepride, from the ara pituitary. rsH increases the production and release of thyroxine (Ti triiodothyronine (T3) from the thyroid. T4 and T3 reduce prodacü T5H which is the basis of the TRH rest (pS4O). The thyroid producests T4, some of which is converted to T3 in the blood or tissues. 85% sil produced this way; 15% is secreted from the thyroid. T3 is five sna active as T4. Most T3 and T4 in plasma is protein bound, maul thyroxine-binding globulin (TaG). It is the unbound portion wh active. T3 and T4 increase much cell metabolism. They are vital to ta growth and mental development. They also increase catecholat effects.

Basic tests Write why you want the test. Different labs do diffeeent8
In general, if you expect hyperthyroidism the most sensitive test is pla
T3 (which is raised): if you expect hypothyroidism the most sensiave
are plasma T4 (lowered) and TSH (raised).

I Plasma T4 (total T4, ie total thyroxine) Method: Collect any time, uncuffed.
Problems of interpretation: (for amiodarone see p544) False I (because bound, as well as free T4 measured): pregnant; oestrcg hereditary thyroid binding globulin excess.
False low: Salicylates; NSAID; phenytoin; corticosteroids; carbamazel thyroid binding globulin deficiency.

2 Plasma total T3 Method: As for 1.

Problems of interpretation: (For amiodarone see p544.) False high: Pregnancy; oestrogens.

False low: Severe infection; post-surgery; post-myocardial isis chronic liver disease; chronic renal failure; propranolol; phenyts salicylates; N5AID; carbamazepine.

3 Plasma basal thyroid-stimulating hormone (TsH)

Indications: Suspected hypothyroidism, risk of hypothyroidism. Method: Collect blood at any time of day.

Problems of interpretation: Normal TSH is <5.7mU/l (some variation). >5.7mU/l with normal T4 indicates partial thyroid fad caused by: Hashimoto’s, drugs (lithium, antithyroids, excess iodine expeceorants), hyperthyroidism treatment, autoimmune disease (tu pernicious anaemia, Addison’s), iodine deficiency, dyshormonogenti

Immunometric assays (aMA) of Ts are now replacing radioimmunoaa and these are so sensitive that low TSHs may be quantified—so vss a are not now needed (p540) when borderline raised thyroid horns levels suggest hyperthyroidism.1 A TSH IMA level <—0.SU/l suggestshy thyroidism. The normal range of T5H widens in the elderly (eg add Is decade over 40yrs to the upper limit).2
If T3 and T4 low, but TSH not raised then diagnose secondary k1 thyroidism (p544). Look for pituitary failure)(PSS8).
I Drug The, But 198927 57—9 2 1 Hay 1988 Endocrinol Metab Ch,, North Am 17 473-SI