How do I know whether my fatigue is caused by my thyroid?
Thyroid blood tests can rule out or confirm whether your thyroid is the cause, but a mildly abnormal TSH alone does not explain fatigue and treating it often does not help. In Hashimoto's, symptoms can persist even when your values are normal.
Fatigue is one of the best-known complaints associated with thyroid problems, but the connection is more complicated than it appears. A clearly underactive thyroid typically comes with other symptoms alongside fatigue, such as a slow heart rate, feeling cold all the time and constipation. That pattern is strong enough to point toward the thyroid and make blood testing worthwhile.
A blood test measures the TSH hormone, which is the control signal for your thyroid. A raised TSH combined with a normal free thyroid hormone (FT4) is called subclinical hypothyroidism. That sounds as though treating it would resolve the fatigue, but it does not. Across 21 randomised trials involving more than 2,000 participants, thyroid hormone treatment (levothyroxine) for subclinical hypothyroidism produced no measurable benefit for fatigue, quality of life or low mood. Treatment therefore makes little sense for this diagnosis when fatigue is the only complaint. An additional consideration: in older people TSH naturally rises with age, meaning subclinical hypothyroidism is sometimes over-diagnosed.
A separate category is the autoimmune condition Hashimoto's thyroiditis. People with Hashimoto's more frequently experience fatigue, forgetfulness, anxiety and sleep problems, even when their TSH and thyroid hormone levels are completely normal. Elevated antibodies (the immune proteins that attack the body's own thyroid tissue) are associated with greater inflammation in the body and a poorer quality of life. In that case the fatigue does not stem from a shortage of thyroid hormone, but appears to be driven by the immune process itself. This also means your TSH value can be normal while you still feel unwell.
Finally, there is subacute thyroiditis, which often follows a viral infection. It is accompanied by pain in the throat, fatigue and muscle aches. This condition typically resolves completely. If fatigue persists despite a normal or treated thyroid, it probably has a different cause. Think of sleep problems, anaemia, stress or another condition. Have your blood values checked by your GP if you are persistently tired, but do not automatically expect a thyroid diagnosis or supplement to be the solution.
Claims are based on a large meta-analysis (n=2192, 21 RCTs) for the treatment effect, and observational studies for the association between Hashimoto's/hypothyroidism and fatigue complaints. TSH as a diagnostic tool is broadly validated.