Endocrine Abstracts

Hypothyroidism exists when function of the hypothalamic–pituitary–thyroid axis is impaired, either with normal or subnormal thyroid hormone levels – compensated and decompensated hypothyroidism. Hypothyroidism is called primary when the abnormality is at the level of the thyroid gland itself and central when the defect is in the hypothalamo-pituituary axis.

Central hypothyroidism, whether congenital or acquired, almost always occurs in the context of panhypopituitarism. It is rarer than primary hypothyroidism and lower thyroxine doses are required compared with primary disease.

Primary congenital hypothyroidism (CH) is the commonest endocrine disorder with a prevalence in Scotland between 1994–2003 of 1/3655.TSH elevation in the newborn period is usually due to CH but may be transient in nature, or of uncertain cause. Of 95 infants undergoing dual ultrasound and isotope scanning in Glasgow 2004–2011 the causes were transient (8), uncertain (7), thyroid dysgenesis (due to ectopia (39), athyreosis (21), hypoplasia (4)) and dyshormonogenesis (20). With newborn TSH screening on days 4–6, notification on days 10–12 and immediate treatment by days 13–15 (unless TSH elevation is mild, e.g. <40 μ/l), the outlook for CH is good. Further improvements might be expected by instituting screening on day 3 so that severely affect infants can begin treatment within 10 days of life; setting the TSH referral cut-off at 8–10 mU/l; adopting a disciplined national diagnostic algorithm to evaluate referred cases, with measurement of venous free T₄, TSH and thyroglobulin combined with dual ultrasound and radioisotope imaging; initial treatment with a thyroxine dose of 50 μg daily in infants weighing ≥2.5 kg and 15 μg/kg per day in infants weighing <2.5 kg followed by weekly review until thyroid function is normalised; and maintenance of free T₄ levels between 15–26 pmol/l and TSH between 0.5–5 mU/l thereafter to avoid both under- and overtreatment.

Primary acquired hypothyroidism is usually due to autoimmune thyroiditis, also called Hashimoto’s thyroiditis (HT). HT is commoner in girls (~ 4:1) and often has a family history (40%). Its frequency in type 1 diabetes, Turner’s syndrome and Down’s syndrome is sufficiently high for annual screening to be justified. We have found school-based capillary TSH screening to be helpful in Scottish children with Down syndrome, in whom minimum prevalence of TSH elevation is ~ 6%. Presentation of HT in the general population is with goitre which may be tender +/− symptoms of hypothyroidism such as pallor, fatigue and weight gain, and occasionally slipped capital femoral epiphysis. The complete replacement dose of thyroxine is 100 μg/m² per day. However, in severe cases initial doses must be low to try and preempt symptoms of the transition from hypo- to euthyroidism. These include tearfulness, fatigue, poor concentration and poor school performance. HT may fluctuate in severity and even remit, so the family should to report early with symptoms suggestive of either under- or over-treatment with thyroxine.