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
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.