| Summary: | Hypothyroidism is a common condition in general practice. Evidence of long-term outcomes and the effectiveness of treatment for subclinical hypothyroidism are lacking. This thesis provides new knowledge to support practice in relation to subclinical hypothyroidism. Methods and Results Several research methodologies were utilised: A cross-sectional study in general practice showed that prevalence of hypothyroidism in Hamilton City was 2.5%, with an overall level of thyroid dysfunction at 3.2%. Examining retrospective laboratory data of thyroid function tests showed that 1 in 6 patients without known thyroid disease are tested within a 12-month period. Focus groups of general practitioners found that patients with a raised thyroid stimulating hormone (TSH) assay posed a dilemma for general practitioners. Retrospective laboratory data and note review of the management of patients with raised TSH found that two percent of patients were managed according to New Zealand Best Practice Advocacy Centre (BPACnz) recommendations. Note review and interviews with patients with central hypothyroidism showed that a first-line TSH strategy was not the cause of delayed diagnosis but indicated a reliance on biochemical results over manifest signs and symptoms. Survival analysis comparing patients with thyroid dysfunction against cardiovascular events and all-cause mortality found that patients with both subclinical and overt hypothyroidism had an increased risk of cardiovascular events [Hazards ratios 1.22 (1.12-1.33) and 1.58 (1.44-1.73)] and death [1.29 (1.17-1.42) and 1.45 (1.31-1.62)] compared to euthyroid individuals after adjusting for age, gender, ethnicity and social deprivation. The increased cardiovascular event risk was greatest in those under 65 years of age with subclinical hypothyroidism [1.26 (1.07-1.49)] compared with those 65+ years [1.14 (1.03-1.27)]. A systematic review of patients with subclinical hypothyroidism treated with thyroxine found that treatment had a positive effect on lipids, BMI, cardiac function and systolic blood pressure in individuals less than 65 years of age with stable subclinical hypothyroidism. Conclusions Subclinical- and overt hypothyroidism are associated with increased cardiovascular morbidity and all-cause mortality with the greatest excess risk in patients with subclinical hypothyroidism who are less than 65 years of age. Treatment with thyroxine should be considered in these patients to reduce cardiovascular risk.
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