- In patients with type 1 diabetes mellitus, routine screening for thyroid dysfunction is essential due to the high prevalence of autoimmune thyroid disease in this population.
- The preferred initial thyroid function test is Thyroid-stimulating hormone (TSH).

- TSH is the most sensitive marker for detecting both overt and subclinical hypothyroidism, which is more commonly associated with type 1 diabetes.
- An abnormal TSH level typically prompts further evaluation with free thyroxine (Free T4) to assess the thyroid hormone status.

- While Free T4 levels are important in determining thyroid hormone concentrations, they are not recommended as the initial screening test because TSH changes occur earlier in thyroid dysfunction.

- Thyroid peroxidase antibodies (TPO Ab) testing helps identify autoimmune thyroiditis (Hashimoto's thyroiditis), but antibody screening is not routinely recommended unless thyroid dysfunction or symptoms suggestive of thyroid disease are present.

- Serum triiodothyronine (T3) is generally not used for routine screening because T3 levels may remain normal in hypothyroidism and are less useful for initial assessment.

- In summary, for routine screening of thyroid function in type 1 diabetes, TSH measurement is recommended as the first-line test.

Reference: Williams Textbook of Endocrinology, 14th Edition, Chapter 26: Disorders of the Thyroid Gland, Page 1058