T4 and TSH

TSH and T4 together assess whether your thyroid gland is producing adequate hormone and whether your pituitary is signaling it correctly. This screen detects hypothyroidism, hyperthyroidism, and pituitary dysfunction—conditions that affect energy, metabolism, and overall health.

When elevated: Elevated T4 with low TSH typically indicates hyperthyroidism (overactive thyroid), causing increased metabolism, rapid heart rate, and anxiety. Elevated TSH with normal or low T4 suggests primary hypothyroidism (underactive thyroid), where the pituitary is working harder to stimulate an underperforming thyroid gland. When low: Low T4 with high TSH indicates primary hypothyroidism, where the thyroid is not producing enough hormone. Low T4 with low TSH is less common and may suggest secondary hypothyroidism (pituitary dysfunction) or acute illness; this pattern warrants further evaluation.

Thyroid hormone regulates metabolic rate, heat production, and recovery; athletes with low thyroid function often report fatigue, sluggish training response, and difficulty maintaining performance despite adequate training. Monitoring T4 and TSH is particularly useful during heavy training blocks or when unexplained performance plateaus or fatigue emerge, as overtraining and caloric restriction can suppress thyroid signaling.