Tests of thyroid function

There is a variety of tests of thyroid function available, some of which are now only of historic interest and others in the province of the endocrinologist rather than the endocrine surgeon. The number of investigations requested should be the minimum necessary to reach a diagnosis and formulate a management plan. Only a small number of parameters needs to be measured as a routine although this may require supplementation or repeat when inconclusive.

Serum thyroid hormones

Serum TSH. TSH levels can be measured accurately down to very low serum concentrations and if the serum TSH level is in the normal range it is redundant to measure the T3 and T4 levels. Interpretation of deranged TSH levels however depends on knowledge of the T3 and T4 values (Table 44.1). In the euthyroid state, T3, T4 and TSH levels will all be within the normal range. Florid thyroid failure results in depressed T3 and T4 levels with gross elevation of the TSH. Incipient or developing thyroid failure is characterised by low normal values of T3 and T4 and elevation of the TSH. In toxic states the TSH level is suppressed and undetectable. Thyroxine (T4) and tri-iodothyronine (T3) are transported in plasma bound to specific proteins (thyroxine-binding globulin, TBG). Only a small fraction of the total (0.03 per cent of T4 and 0.3 per cent of T3) is free and physiologically active. Assays of both total and free hormone are available but the total values depend on the level of circulating proteins which are affected by the level of circulating oestrogen. Thus, pregnant women and those on the oral contraceptive pill have elevated total T4 and T3 levels without evidence of toxicity. The free hormone levels are unaffected. Similarly some patients have low levels of TBG either as a primary phenomenon or secondary to a reduction in serum protein levels as a result of systemic or liver disease and the total level of circulating hormone may be low. For these reasons the free levels are more meaningful. Highly accurate radioimmunoassays of free T3 and free T4 are now routine. T3 toxicity (with a normal T4) is a distinct entity and may only he diagnosed by measuring the serum T3, although a suppressed TSH level with a normal T4 is suggestive.

Various combinations of these tests are used in different laboratories. An appropriate combination is to establish the functional thyroid status at initial assessment, with TSH supplemented by free T4, and T3 evaluation when TSH is abnormal.

Isotope scanning (Fig. 44.7)

The uptake by the thyroid of a low dose of either radiolabelled iodine (123I) or technetium-99m (99”m’Tc, which is normally taken up like 123I) will demonstrate the distribution of activity in the whole gland. This test is inappropriate for distinguishing benign from malignant lesions because the majority (80 per cent) of  cold swellings is benign and some (5 per cent) functioning or warm swellings will be malignant. Its principal value is in the toxic patient with a nodule or nodularity of the thyroid. Localisation of overactivity in the gland will differentiate between a toxic nodule with suppression of the remainder of the gland and toxic multi-nodular goitre with several areas of increased uptake with important implications for therapy. Routine isotope scanning is unnecessary.

Whole body scanning is used to demonstrate metastases but the patient must have all normally functioning thyroid tissue ablated either by surgery or by ablation with high-dose radioiodine before the scan is performed because thyroid cancer cannot compete with normal thyroid tissue in the uptake of iodine.

Thyroid autoantibodies

Serum titres of antibodies against thyroid peroxidase and thyroglobulin are useful in determining the cause of thyroid dysfunction and swellings. Autoimmune thyroiditis may be associated with thyroid toxicity, failure or euthyroid goitre. Titres of greater than 1:100 are considered significant but a proportion of patients with histological evidence of lymphocytic (autoimmune) thyroiditis is seronegative.