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Thyroid Hormones are iodine-containing signalling molecules, principally thyroxine (T4) and triiodothyronine (T3), secreted by the thyroid gland under negative-feedback control of the hypothalamic-pituitary-thyroid axis. T4 serves primarily as a prohormone; peripheral deiodinase enzymes convert it into the biologically active T3, which binds nuclear thyroid hormone receptors to regulate whole-body metabolism, thermogenesis, cardiac function, and neurological development.
Clinically, thyroid function is assessed primarily via serum TSH, which rises in hypothyroidism and falls in hyperthyroidism, offering a sensitive proxy for the body's net thyroid hormone activity.
How it works
The hypothalamic-pituitary-thyroid (HPT) axis governs thyroid hormone production through a classical negative-feedback loop. The hypothalamus releases thyrotropin-releasing hormone (TRH), prompting the anterior pituitary to secrete thyroid-stimulating hormone (TSH). TSH drives thyroid follicular cells to synthesise and release T4 and T3 into the bloodstream. When circulating T3 reaches sufficient concentration, it suppresses both TRH and TSH secretion, closing the loop and preventing overproduction 1.
Despite the thyroid gland secreting far more T4 than T3, roughly 80% of circulating T3 is produced not by the gland itself but by peripheral deiodination. Type 1 and type 2 deiodinase enzymes (DIO1, DIO2) in the liver, kidneys, skeletal muscle, and brain strip one iodine atom from T4 to generate T3. Type 3 deiodinase (DIO3) performs the opposite function, inactivating T3 and T4 by producing the metabolically inert reverse T3 (rT3), providing a tissue-level brake on thyroid hormone activity 2.
Once inside target cells, T3 binds nuclear thyroid hormone receptors (TRs), which interact with thyroid hormone response elements on DNA to modulate gene transcription. T3 is three to four times more potent than T4 at these receptors 1. The downstream effects span the entire body: T3 stimulates mitochondrial biogenesis and uncoupling protein expression in skeletal muscle and brown adipose tissue, raising resting oxygen consumption and heat production 4. This molecular signalling explains why thyroid hormone status has such pervasive effects on energy balance, body composition, and cardiovascular function.
Example
A strength athlete presenting with fatigue, cold intolerance, and unexplained weight gain undergoes blood testing. Results reveal a TSH above the reference range, indicating the pituitary is working harder than normal to coax hormone output from an underactive gland. T3-mediated mitochondrial activity has dropped, reducing resting metabolic rate and blunting the skeletal muscle response to training. Levothyroxine normalises TSH over several weeks and metabolic function gradually recovers.
The scenario illustrates that suboptimal thyroid function mimics overtraining syndrome, making hormone status a primary diagnostic consideration before attributing unexplained fatigue to training load.
Why it matters
Thyroid hormones occupy a central position in metabolic regulation, and deviations in either direction carry measurable costs. Hypothyroidism reduces resting metabolic rate by up to 30%, promotes dyslipidaemia, and impairs cardiac output; hyperthyroidism drives tachycardia, elevates atrial fibrillation risk, and accelerates bone resorption 1. A large individual participant data meta-analysis found that free T4 values above the 85th percentile were associated with more than a 5% increase in 10-year composite cardiovascular risk, while TSH in the 60th to 80th percentile range corresponded to the lowest all-cause and cardiovascular mortality 3.
For anyone focused on body composition or endurance performance, the deiodinase system adds a layer of complexity that serum TSH alone does not capture. Local DIO2 and DIO3 activity lets individual tissues modulate intracellular T3 independently of circulating hormone levels 2. A minority of patients whose TSH is well-controlled on levothyroxine continue to report residual symptoms, a pattern linked to genetic variation in DIO2 that impairs local T4-to-T3 conversion; this is the scientific basis for the ongoing debate over T4/T3 combination therapy.
What is the difference between T3 and T4?+
T4 (thyroxine) is the thyroid gland's primary secretion and functions mainly as a prohormone. Peripheral deiodinase enzymes convert T4 into T3 (triiodothyronine), the biologically active form. T3 is three to four times more potent than T4 and is responsible for the downstream effects on metabolism, thermogenesis, and gene transcription {{cite:10.1038/s41574-019-0218-2}}.
What do thyroid hormones do for metabolism and energy?+
Thyroid hormones, acting principally through T3, stimulate mitochondrial biogenesis, raise resting oxygen consumption, and increase thermogenesis. They also regulate cardiac contractility, protein synthesis, and glucose metabolism. Hypothyroidism lowers metabolic rate by up to 30%, contributing to fatigue, weight gain, and reduced exercise tolerance {{cite:10.1152/physrev.00030.2013}} {{cite:10.1210/js.2018-00423}}.
What are the symptoms of low thyroid hormone (hypothyroidism)?+
Hypothyroidism presents with fatigue, cold intolerance, unexplained weight gain, dry skin, constipation, slowed heart rate, and cognitive dulling. These symptoms reflect reduced T3-mediated gene transcription across multiple organ systems. A raised TSH is the primary diagnostic marker, as the pituitary increases output to compensate for insufficient thyroid hormone {{cite:10.1152/physrev.00030.2013}}.
What is a normal TSH level and why does it matter?+
TSH reference ranges typically span 0.4 to 4.0 mIU/L, though optimal values for minimising cardiovascular and mortality risk appear narrower. A large meta-analysis of over 134,000 participants found that TSH in the 60th to 80th percentile range was associated with the lowest all-cause and cardiovascular mortality, highlighting the importance of within-range optimisation {{cite:10.1016/s2213-8587(23)00227-9}}.
