Tests Regarding the Thyroid Gland Functioning

Plasma total thyroxine (T4)

 * Normal plasma concentration: 60 - 150 nmol/l
 * Was widely used in the past as a test of thyroid function
 * Disadvantage: dependent on concentration of binding protein (TBG)

Plasma total triiodothyronine (T3)

 * Normal plasma concentration 1.0 - 2.9nmol/l
 * Concentration nearly always raised in hyperthyroidism (to greater extent than t4)
 * May be normal in hypothyroidism due to preference in production of T3 in thyroid and then peripherally converted to T4
 * Disadvantage: dependent on concentration of binding protein (TBG)

Reasons for abnormal concentrations of thyroxine-binding globulin (TBG)

Free T4 test

 * Normal plasma concentration 10 - 2 5 pmol/l
 * Measure only unbound, active thyroxine
 * Rountine clinical use in laboratories
 * Measures thyroxine-binding globulin (TBG) too

Free T3 test

 * Normal plasma concentration 3.5 - 7.5 pmol/l

Thyroid Stimulating Hormone (TSH) measurement

 * Normal plasma concentration 0.3 - 3.5 mU/L.
 * Levels of TSH can differentiate between hypothyroidism, hyperthyroidism, and euthyroidism.
 * Eg. In case suspicion of primary thyroid disease; but if plasma TSH concentration is normal, patient is euthyroid. To diagnose primary hypothyroidism, tsh concentrations should be greatly increased
 * TSH measurement is most sensitive as they increase above normal range before T4 fall below
 * Accurate diagnosis however requires at least 2 tests, eg. TSH with free T4 / T3

Plasma free T4

Thyrotrophin- Releasing Hormone (TRH) test

 * Obsolete as modern sensitive basal TSH immunoassays preferred, except for investigation of hypothalamic-pituitary dysfunction.
 * To test: plasma TSH measured before, 20 min, and 60 min after giving the patient 200 µg of TRH i.v. Normally, TSH concentration increases by 2 - 20 mu/l in 20 min, then reverts to basal level at 60 min.
 * Delayed (higher concentration at 60 min than at 20 min) TSH response to TRH is characteristic for hypothalamic disease.

‘Sick euthyroid syndrome’ in patients with non-thyroidal illness but other systemic diseases (eg infections, malignancy, myocardial infarction, post-surgery) can have an apparently low total and free T4 and T3 with a normal or low basal TSH. Levels are usually only mildly below normal and are thought to be mediated by interleukins IL-1 and IL-6.

They are caused by:


 * reduced concentration and affinity of binding proteins.
 * decreased peripheral conversion of T4 to T3 with more rT3 (reverse T3 blocks action of normal T3).
 * non-thyroidal influences on the hypothalamic-pituitary-thyroid axis (eg by cortisol) to inhibit TSH production.
 * increased plasma free fatty acids displacing thyroid hormones from their binding sites.