Objective: To investigate the association between subclinical hypothyroidism and preterm contractions. Methods: Prospective observational study among women at 23+0/7 and 34+6/7 weeks of gestation, with no known thyroid function abnormality, and preterm uterine contractions (PTC). All patients underwent laboratory evaluation of Thyroid Stimulating Hormone (TSH) and Free Thyroxin (FT4). Patients with and without PTC were compared. Results: No association was found between PTC and subclinical hypothyroidism. Rate of spontaneous preterm delivery (PTD) was comparable between women with abnormal and normal thyroid function tests. Excluding indicated PTD, patients in the study group had a higher rate of spontaneous PTD (24.7% versus 9.6%, p = 0.03). Patients with past PTD and preterm contractions had higher rates of hypothyroxinemia compared with patients without past PTD (54.6% versus 19.0% and 31.2%, p = 0.001), and patients with past PTD (regardless of the presence or absence of PTC) had higher rate of subclinical hypothyroidism compared with patients with PTC and without PTD (59.1% and 66.7% versus 31.6%, p = 0.017). Conclusions: No association was found between PTC and subclinical hypothyroidism in the entire cohort, except for patients with preterm contractions and a history of past PTD. This specific group of patients might benefit from thyroid function evaluation.