Although T3 is the active thyroid hormone-that is, the form of the hormone which binds to intracellular receptors - the usefulness of serum T3 measurements in patient care has been the subject of much debate. The thyroid gland secretes much more T4 than T3. And of the total plasma concentration of T3, only around 20% has come from thyroid secretion. The remainder arises from conversion of T4 to T3 in peripheral tissues. At Glasgow Royal Infirmary we offer total T4 and TSH as the first line analyses when a doctor asks for thyroid function tests. Although no longer part of a first line test, T3 is routinely measured in any patient with suspected hyperthyroidism, as well as patients taking antithyroid drugs such as carbimazole.
Prevalence data show that over 99% of all cases of hypothyroidism and hyperthyroidism result from disorders affecting the thyroid gland itself. The TSH secreting cells of the anterior pituitary respond to changes in circulating thyroid hormone concentrations. In primary hypothyroidism, therefore, the pituitary reacts to the deficiency in circulating thyroid hormones by increasing its production of TSH. Thus, a raised serum TSH concentration is diagnostic of primary hypothyroidism. Conversely, if thyroid production of T4 and T3 is increased pituitary production of TSH will be suppressed. Thus, in a previously untreated
patient with symptoms of thyrotoxicosis an undetectable serum TSH concentration confirms the diagnosis of primary hyperthyroidism. These simple key concepts are outlined in figure 1. Hyperthyroidism due to a pituitary adenoma secreting TSH is extremely rare, so much so that even an endocrinologist is unlikely to encounter a case during his or her career. Isolated secondary hypothyroidism, in which pituitary secretion of TSH is impaired, is also rare but can occur more commonly as part of generalised hypopituitarism.
Fig 1 - The main factor regulating thyroid function is the negative feedback control that T4 (thyroxine) and T3 (tri-iodothyronine) exert on TSH (thyroid stimulating hor,none) secretion by the pituitary gland. Thus, in primary hyperthyroidism the serum TSH concentration is low as its production is suppressed by the high concentrations of T4 and T3. In primary hypothyroidism the serum TSH concentration is raised owing to the lack of the normal inhibition of its production by T4 and T3
We emphasise that reference ranges (or normal ranges) for test results are compiled from a healthy population without evidence of thyroid disease. These reference ranges clearly help in initially diagnosing disease in patients with suspected thyroid hyperfunction or hypofunction. In patients undergoing treatment, however, apparently normal results and those which fall outside reference limits may be interpreted quite differently. This is highlighted in the case presented here. Our laboratory's reference ranges are shown in the table.
| Reference serum concentrations for thyroid hormones* | |
|---|---|
| Thyroxine (T4) | 55-144 nmol/l |
| Thyroid stimulating hormone (TSH) | 0.35-5.0 mU/l |
| Tri-iodothyronine (T3) | 0.9-2.8 nmol/l |
| Free thyroxine (free T4) | 9-24 pmol/l |
| *Glasgow Royal Infirmary. | |
Her TSH concentration is less than the detection limit of the assay, and this, together with the raised total T4 concentration and clinical history, confirms the diagnosis of primary hyperthyroidism. Her serum T3 concentration was 7.9 nmol/l. Note that her serum T4 concentration is only 50% higher than the upper limit of the reference range but that her serum T3 concentration is more than twice the upper limit of normal. A characteristic feature of primary hyperthyroidism is a proportionately greater rise in serum concentrations of T3 than of T4.
At the thyroid clinic of the local hospital the clinical diagnosis of Graves' disease was confirmed by the results of autoantibody studies. Graves' disease is the commonest form of hyperthyroidism and is an autoimmune condition in which antibodies to the TSH receptor on the surface of thyroid cells mimic the action of the pituitary hormone.
The patient was prescribed carbimazole 40 mg daily and was seen again five weeks later. She had noted obvious improvement in her general wellbeing, but she was still restless and continued to have a tremor and to be heat intolerant. Thus, she was still clinically thyrotoxic. The results of biochemical thyroid function tests at this consultation showed serum con- centrations of T4 of 15 nmol/l; T3, 1.8 umol/l; and TSH 18 mU/l.
Fig 2 - In Graves' disease autoantibodies to the TSH receptor stimulate the thyroid gland when they bind to their antigen. This stimulation of the thyroid is not influenced by the normal negative feedback mechanism that controls TSH secretion. Thus, patients with Graves' disease can be euthyroid with low concentrations of TSH as their thyroid glands are being stimulated by the antibodies to TSH that are charactersitic of the disease.
The biochemical results show that she was now becoming hypothyroid.
Why is there a discrepancy between clinical and biochemical status?
Carbimazole acts by inhibiting the synthesis of T4 and T3. The patient was taking a large loading dose of the drug, which rapidly blocks thyroidal T4 and T3 biosynthesis and resulted in the profound drop in serum concentrations of T4 and T3. These hormones have a short half life and disappear from blood quickly when biosynthesis is inhibited. The pituitary gland detected this reduction in circulating thyroid hormone quickly and responded by increasing TSH production.
However, not all tissues in the body respond at the same rate as the pituitary to the fall in concentrations of T4 and T3 in the plasma. Some tissues, such as skeletal muscle and the heart, continue to respond to the previously high concentrations of T4 and T3 and so the clinical symptoms of hyperthyroidism persist.
Although the patient was still clinically thyrotoxic, the biochemical results clearly indicated to the physician looking after her that her dose of carbimazole should be reduced. She was prescribed a maintenance dose of carbimazole of 10 mg daily and over the following month she became clinically euthyroid.
Although T4 and T3 concentrations are within their reference ranges, TSH concentration is somewhat low.
Why might this be so?
This is a common finding in patients with Graves' disease who are being treated with antithyroid drugs. In this case serum TSH concentration alone is not a reliable indicator of the patient's thyroid status. What might be happening is outlined in figure 2. The reduced carbimazole dose is partly blocking the synthesis of T4 and T3, and she now has sufficient thyroid hormone in her plasma to maintain her in a euthyroid state. The stimulus for T4 and T3 production is coming from two sources: TSH produced by the pituitary, and the thyroid stimulating immunoglobulins. Because of the presence of these autoantibodies the pituitary does not need to produce as much TSH as normal to maintain T4 and T3 concentrations within reference limits.
Withdrawing the carbimazole has allowed the thyroid stimulating immunoglobulins to cause excessive production of T4 and T3 from the thyroid gland. The circulating concentration of T4 has increased from 79 nmol/l to 129 nmol/l, although this value still remains within the reference range. She has a more obvious increase in T3 concentration. The increase in thyroid hormone concentrations has resulted in the complete suppression of TSH
FIG 2-In G raves' disease autoantibodies to the TSH receptor stimulate the thyroid gland when they bind to their antigen. This stimulation of the thyroid is not influenced by the normal negative feedback mechanism that controls TSH secretion. Thus, patients with Graves' disease can be euthyroid with tow concentrations of TSH as their thyroid glands are being stimulated by the antibodies to TSH that are characteristic of the disease production by the pituitary. The undetectable TSH concentration is the main indicator of the recurrence of the thyrotoxicosis because the pituitary responds faster than other tissues to the changes in circulating concentrations of thyroid hormones. The clinical signs and symptoms of the recurring disease take longer to develop.
The patient was then overtly thyrotoxic, although from the laboratory's point of view there had not been any dramatic change in the results of thyroid function tests. But the patient's tissues were clearly affected by the increased concentrations of thyroid hormones to which they were exposed, giving rise to clinical manifestations of the disease.
This pattern of results seen in isolation, perhaps when a patient is initially diagnosed as being hyperthyroid, would be labelled T3 toxicosis. TSH concentration is suppressed, total T3 concentration is raised, but total T4 concentration lies within the reference range. Such a pattern may simply represent a stage in the development of the classic biochemical results of primary hyperthyroidism rather than a separate disease state. Measurement of free T4 concentration may well be of value. In this case a raised free T4 concentration of 40 pmol/l further confirmed the clinical findings.
Another possible treatment option, radioiodine treatment, is usually avoided in patients who intend to have children. When it is used, patients must be warned to avoid pregnancy or fathering a child for at least six months.
Before surgery she was again rendered euthyroid with carbimazole. This treatment reduces the risk of thyrotoxic crisis after the operation. The final decision to undergo surgery was taken when she was clinically euthyroid.
Four weeks after the operation she was reviewed at the outpatient department. Her thyroidectomy scar had healed well, but she complained of being excessively tired. This was her only symptom, and she did not have signs suggestive of hypothyroidism. She had no signs or symptoms of hypocalcaemia, and her serum calcium concentration was normal. Thyroid function tests then showed serum concentrations of T4 of 37 nmol/l, T3 1.4 nmol/l, and TSH 47 mU/l.
Hypothyroidism has an insidious onset, and in its early stages non-specific symptoms such as excessive tiredness may be its only manifestation. The patient showed a significantly reduced serum T4 concentration, but, importantly, her serum TSH concentration was significantly increased. Too much functioning thyroid tissue was removed at surgery, and this left her hypothyroid.
Note that her T3 concentration remained within the reference range. This is a typical finding in primary hypothyroidism. Indeed, T3 measurements have little value in the diagnosis and monitoring treatment of patients with primary hypothyroidism.
Four months later she was clinically euthyroid while taking a replacement dose of thyroxine of 100 mg daily. At that stage her serum concentration of T4 was 167 nmol/l and of TSH 0.17 mU/I.
Should her replacement dose of thyroxine be reduced?
The goal of treatment in primary hypothyroidism is to restore patients to the euthyroid state. This patient's serum T4 concentration was raised while TSH concentration was below the reference range. A slight reduction in circulating TSH concentration is unlikely to indicate overtreatment with thyroxine. However, if the serum TSH concentration was to become persistently undetectable in the presence of a raised T4 concentration the thyroxine dose should be reduced.
To achieve TSH concentrations within the normal range many patients require a dose of thyroxine that increases their serum T4 concentrations just above the reference range. This is necessary to compensate for the lack of endogenous production of T3 by the thyroid because all the T3 must now be derived by peripheral conversion of T4.