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CDHB

Assessment of Thyroid Function

Abnormalities in thyroid function tests, not requiring treatment are often observed in patients with systemic non-thyroidal illness. These abnormalities are often referred to as the 'sick euthyroid syndrome'. Therefore thyroid function should not be assessed in seriously ill patients unless there is a strong suspicion of thyroid dysfunction. In pregnant patients, measurement of total T4 is more accurate for assessing thyroid function. In acutely unwell hospital patients, total T4 is also recommended (use Endolab request form for total thyroid hormone levels).

In acute non-thyroidal illness, conversion of T4 to T3 is reduced, and T3 measurements are usually unhelpful - particularly in ICU where the lowest T3 levels are seen.

• High TSH with Free T4 normal - These findings are consistent with sub-clinical hypothyroidism and may be associated with a small goitre and positive thyroid antibodies. Patients with a sustained TSH >10 milliunit/L usually have primary hypothyroidism requiring treatment with thyroxine. If the patient is elderly or has ischaemic heart disease, start thyroxine 25-50 microgram per day, increase by 25 microgram every 4-8 weeks; if young, start 100 microgram daily; repeat TFTs in 6 weeks. In the recovery phase after acute non-thyroidal illness, TSH may transiently show a slight elevation, usually <6 milliunit/L. TSH and Free T4 should be repeated after 6-8 weeks.
• Low (or low/normal) Free T4 and normal TSH - These results are often seen in serious non-thyroidal illness but also raise the question of secondary hypothyroidism. Repeating the thyroid function tests after 6-8 weeks is recommended unless there is a high suspicion of pituitary/ hypothalamic disease. In the latter case, screening for evidence of other pituitary dysfunction may be necessary, i.e., plasma prolactin, plasma cortisol at 0800 hours, LH/FSH and testosterone or oestradiol. Consult Thyroid Physicians or Endocrinology for advice.
• High Free T4 and suppressed TSH - Thyrotoxicosis is likely, particularly if accompanied by a goitre and signs of hyperthyroidism. Request T3 levels, thyroid antibodies, baseline FBC and LFTs. A radionuclide thyroid scan is helpful to distinguish thyroiditis from toxic nodular disease or Graves' disease. Consult Thyroid Physicians. Treat thyrotoxicosis with carbimazole 5-30 mg daily depending on severity; repeat TFTs in 4-6 weeks. Warn all patients about the rare but serious complication of agranulocytosis. Consider beta-blocker for symptom control. If radionuclide scan shows no uptake, stop carbimazole, treat as thyroiditis.

  Patients with suppressed TSH (<0.2 milliunit/L) and normal Free T4/T3 have sub-clinical thyrotoxicosis; recommend general practitioner repeats TFTs in 2 months.

• Amiodarone - This is a frequent cause of thyroid function abnormalities. Conversion of T4 to T3 is reduced and with long-term administration Free T4 may be modestly elevated with TSH and T3 normal. The high iodine content of amiodarone may also precipitate either thyrotoxicosis (suppressed TSH) or hypothyroidism (elevated TSH). Hypothyroidism is treated with cautious thyroxine replacement therapy. Thyrotoxicosis can be difficult to treat - consult Thyroid Physicians.
• For further guidance on treatment of thyroid disorders, refer to HealthPathways.

Topic Code: 1365