We present the case of a 57-year-old lady who had a delayed diagnosis of central hypothyroidism on a background of Grave’s thyrotoxicosis and a partial thyroidectomy. During the twenty years following her partial thyroidectomy, the patient developed a constellation of symptoms and new diagnoses, which were investigated by numerous specialists from various fields, namely rheumatology, renal and respiratory. She developed significantly impaired renal function and raised creatine kinase (CK). She was also referred to a tertiary neurology service for investigation of myositis, which resulted in inconclusive muscle biopsies. Recurrently normal TSH results reassured clinicians that this did not relate to previous thyroid dysfunction. In 2015, she developed increased shortness of breath and was found to have a significant pericardial effusion. The clinical biochemist reviewed this lady’s blood results and elected to add on a free T4 (fT4) and free T3 (fT3), which were found to be <0.4 pmol/L (normal range (NR): 12–22 pmol/L) and 0.3 pmol/L (NR: 3.1–6.8 pmol/L), respectively. She was referred urgently to the endocrine services and commenced on Levothyroxine replacement for profound central hypothyroidism. Her other pituitary hormones and MRI were normal. In the following year, her eGFR and CK normalised, and her myositis symptoms, breathlessness and pericardial effusion resolved. One year following initiation of Levothyroxine, her fT4 and fT3 were in the normal range for the first time. This case highlights the pitfalls of relying purely on TSH for excluding hypothyroidism and the devastating effect the delay in diagnosis had upon this patient.Learning points: class="unordered" style="list-style-type:disc">Isolated central hypothyroidism is very rare, but should be considered irrespective of previous thyroid disorders.If clinicians have a strong suspicion that a patient may have hypothyroidism despite normal TSH, they should ensure they measure fT3 and fT4.Laboratories that do not perform fT3 and fT4 routinely should review advice sent to requesting clinicians to include a statement explaining that a normal TSH excludes primary but not secondary hypothyroidism.Thyroid function tests should be performed routinely in patients presenting with renal impairment or a raised CK. class="head no_bottom_margin" id="__sec2title">BackgroundCentral hypothyroidisim is rare with an incidence of 1 in 80 000–120 000 individuals (). It is characterised by insufficient thyroid gland stimulation by TSH, resulting from hypothalamic or pituitary dysfunction. It is rarely isolated and occurs more commonly in conjunction with other pituitary hormone deficiencies, as well as neurological symptoms and signs resulting from the causative lesion. Symptoms tend to be similar to primary thyroid dysfunction, but according to the literature may be milder ().In adults, the commonest causes are pituitary macroadenomas, pituitary surgery or irradiation (), but there are increasing examples of genetic causes causing isolated central thyroid dysfunction.With the use of serum TSH as an initial screening test for thyroid dysfunction, the diagnosis of central hypothyroidism may be delayed or even missed because most of these patients have normal or low TSH. Hence, many guidelines, including the British Thyroid Association, suggest that ‘if laboratories are unable to identify those specimens that specifically require the measurement of both serum TSH and fT4, then it would be prudent to measure serum TSH and fT4 on all specimens rather than embark on a first-line serum TSH strategy’. They suggest that ‘Measurement of serum TSH alone is appropriate after the first investigation in the sequential follow-up of individuals who have not been treated for thyroid disorders and who may be at risk of developing thyroid dysfunction’ ().Reporting of cases like this is important in order to raise awareness of this condition to general practitioners and other physicians who may be falsely reassured by a normal TSH result. As a result of this case, our biochemistry department has changed its advice to GPs about what to do in the case of a normal TSH : ‘TSH within limits excludes primary (but not secondary) hypothyroidism. If there is high clinical suspicion of hypothyroidism, please contact laboratory clinical staff to discuss further tests’.
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