Can hypothyroidism lead to fat gain?

Yes, a less active thyroid will reduce metabolic rate and can cause some weight gain. It is not a lot of weight gain though, and a subactive thyroid is not an excuse for obesity (if hypothyroidic, please see a doctor for medication; it burns fat)

A little. Thyroid problems play a negligible role in obesity.

The American Thyroid Association states:

In general, 5-10 pounds of body weight may be attributable to the thyroid, depending on the severity of the hypothyroidism.[1]

Most of the weight gain directly associated with thyroid issues is negligible[2][3]. In fact, the average weight loss from treatment was 4.3kg, most of which was attributed to the excretion of excess body water[4][5]

Pre-existing obesity, a diagnosis of Graves' disease and prior weight loss independently predicted weight gain and weight continued to rise with time. [2]

References

  1. Thyroid.org: Thyroid and Weight
  2. Dale J, et al Weight gain following treatment of hyperthyroidism . Clin Endocrinol (Oxf). (2001)
  3. Fox CS, et al Relations of thyroid function to body weight: cross-sectional and longitudinal observations in a community-based sample . Arch Intern Med. (2008)
  4. Karmisholt J, Andersen S, Laurberg P Weight loss after therapy of hypothyroidism is mainly caused by excretion of excess body water associated with myxoedema . J Clin Endocrinol Metab. (2011)
  5. Crocker MK, Kaplowitz P Treatment of paediatric hyperthyroidism but not hypothyroidism has a significant effect on weight . Clin Endocrinol (Oxf). (2010)