Symptoms and Signs

 Few patients with subclinical hypothyroidism present with classical symptoms and signs of hypothyroidism. In the Colorado thyroid disease prevalence study, more patients with subclinical hypothyroidism than euthyroid individuals reported multiple symptoms, such as dry skin, poor memory, slow thinking, muscle weakness, constipation, and tiredness. However, the association was weak with low sensitivities (3– 28%) and positive predictive values (8– 12%) for the individual symptoms, such that only a minority of patients with subclinical hypothyroidism had the symptoms while many of the individuals with these symptoms had normal thy roid function. Similarly, another case– control study found that patients with subclinical hypothyroidism and euthyroid controls have similar frequencies of symptoms of hypothyroidism, except tiredness which was more prevalent in patients with subclinical hypothyroidism.

Laboratory Investigations

The diagnosis of subclinical hypothyroidism is based on laboratory thyroid function tests: a serum TSH level above the laboratory reference range together with a normal serum fT4 level is suggestive of the diagnosis. A serum TSH level of 4.5– 5 mU/ L is widely accepted as the upper limit of the reference range, although there is a considerable debate whether this should be set lower at 2.5 mU/ L.

Several physiological and demographic factors affect serum TSH levels that may influence the interpretation of laboratory thyroid function tests. First, the presence of a diurnal variation in TSH secretion with peak levels at night and trough levels in the afternoon means that the timing of the blood test may affect the TSH result. Second, TSH levels progressively rise with age, possibly reflecting an adaption of the hypothalamus- pituitary- thyroid axis to ageing, and therefore some experts advocate the use of age specific TSH reference ranges. Third, ethnicity influences serum TSH; for example, the levels tend to be higher in Caucasians than in Blacks or Hispanics. Finally, an individual’s TSH levels over time fluctuate within a much narrower range than the population reference range. This suggests that each individual has their own TSH set point, and any deviation from that set point may be abnormal for the individual even if the level is within the population reference range.

It is also important to distinguish subclinical hypothyroidism from other conditions associated with a raised serum TSH and normal fT4 (Table 1). The presence of heterophile or human anti- animal antibodies can result in spuriously high serum TSH by interfering TSH assay. Macro- TSH (a biologically inert large molecular complex of TSH and immunoglobulin G) is another cause of spurious elevation of serum TSH, and accounts for about 1.5% cases of apparent subclinical hypothyroidism. Patients recovering from non- thyroidal illness and thyroiditis (subacute, painless, or postpartum) often show transient rise in TSH levels. Likewise, patients with untreated Addison’s disease may have a raised TSH, which normalizes after glucocorticoid replacement. Patients on levothyroxine replacement for overt hypothyroidism frequently have raised TSH due to inadequate dose, non- concordance, malabsorption, and concomitant use of drugs impairing levothyroxine absorption (e.g. iron, calcium, and cholestyramine) or increasing levothyroxine clearance (e.g. phenytoin, phenobarbital, and rifampicin). Resistance to thyroid hormone due to mutations in the thyroid hormone receptor β gene is often associated with raised TSH with variable levels of fT4. However, most patients with this condition will have fT4 above or towards the upper end of the reference range; in contrast, fT4 levels tend to be towards the lower half of the reference range in subclinical hypothyroidism. Furthermore, as resistance to thyroid hormone is an autosomal dominant condition, first degree relatives of the patient may show similar thyroid function test abnormalities.

Table1. Conditions simulating subclinical hypothyroidism with a raised TSH and normal free T4

In patients with subclinical hypothyroidism, the presence of thyroid peroxidase antibodies (TPO- Ab) not only confirms autoimmunity as the aetiology, but also provides a prognostic value for predicting progression to overt hypothyroidism. Thyroid ultra sound may show hypoechogenicity or heterogeneous echogenicity in autoimmune thyroid disease leading to subclinical hypothyroidism; however, a routine thyroid ultrasound is unnecessary in the diagnostic work- up of subclinical hypothyroidism.

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قسم التطوير يقدم دورة عن أخلاقيات المهنة ضمن برنامج تأهيل المنتسبين الجدد

ضمن برنامج (وفاءً للدماء).. المجمع العلمي يقيم محفلًا قرآنيًّا في محافظة المثنى

قسم الشؤون الفكرية يرفد المكتبة المركزية في ديالى بإصدارات معرفية

قسم التطوير يبحث مع ديوان الوقف الشيعي آفاق التعاون في مجال التدريب والتطوير المؤسسي

المزيد

الآخبار الصحية

خبير يكشف 5 حقائق طبية صادمة وغير معروفة

ليس الأسود ولا الحيتان.. "كائن صغير" هو أخطر قاتل للبشر!

بارقة أمل لمرضى "الإيدز".. علاج خلوي ينجح في كبح الفيروس

ظاهرة مقلقة تتوسع.. "المستنشقات" خطر صامت يهدد المراهقين

المزيد

الآخبار التكنلوجية

اكتشاف مادة في الثوم تحسن قوة العضلات وتبطئ شيخوختها

تطور الحياة على الأرض المبكرة اعتمد على معدن نادر

باحثون صينيون يطوّرون بطارية مائية قد تعمل 300 عام

الذكاء الاصطناعي..هل يصبح متهما جنائيا في الجرائم والانتحار؟

المزيد

مواضيع ذات صلة

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Enzyme-Linked Immunosorbent Assays