Subclinical hypothyroidism (SHypo) is characterized by elevated serum TSH and thyroid hormone levels at the lower limit but within their respective reference range.
It is necessary to distinguish between patients with mildly increased serum TSH levels (59 mU/L) and patients with more severely increased serum TSH levels (10 mU/l or higher). About 75% of all SHypo patients have mild disease.
The high prevalence (between 4 and 20% of the adult population) and the various implications of SHypo require the need to establish a correct diagnosis, clinical assessment and treatment of this disorder.
The evaluation of transient and false causes of mild increases in TSH should be excluded before treating SHypo.
Subclinical hypothyroidism may be progressive or reversible. The annual rate of progression to overt disease is particularly increased (4.3%) in women with elevated serum TSH and anti-thyroid antibodies.
Important cardiovascular and metabolic effects may develop in long-term untreated SHypo.
Elderly Subjects with a TSH level of 7 mU/L or greater have a higher risk of heart failure events than euthyroid subjects.
There are some discrepancies in epidemiological data with reference to the risk of coronary heart disease (CHD) in patients with SHypo. A recent meta-analysis analyzed the individual data of 55 287 participants from 11 prospective cohorts. This analysis confirms that the risk of both CHD and mortality due to CHD were significantly increased in participants with TSH levels of 10 mU/L or greater. These results strongly support the association between CHD and SHypo in patients with TSH of 10 mU/L or greater.
Treatment of SHypo is recommended for all patients with SHypo with serum TSH levels of 10 mU/L or greater because these patients may have a significantly increased risk of progression to overt hypothyroidism, are more frequently symptomatic and may have a higher chance of dyslipidemia and cardiovascular dysfunction with an increased risk of CHD events, CHD mortality and CHF.
No consensus exists on the clinical significance and treatment of the mild form of thyroid failure. The available data suggest that treatment of mild SHypo should be personalized. Clinicians should consider the degree of TSH increase, the patients age, the risk of progression to overt disease, the quality of life, the cognitive, metabolic and cardiovascular risk factors and the presence of associated co morbidities.
Declaration of interest: The author declares that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.