Thyroid Function Tests
Thyroid function tests measure thyroid-stimulating hormone with free T4 and, when needed, free T3 and thyroid antibodies; they form part of assessing weight change, but thyroid hormone is not a weight-loss treatment.
Plain English
Patient-Friendly Explanation
Thyroid function tests are blood tests that show how well the thyroid gland is working. The usual first test is thyroid-stimulating hormone, often with free T4, the main thyroid hormone circulating in the blood; free T3 and thyroid antibodies are added when the result is abnormal. These tests are part of the assessment of unexplained weight change because an underactive thyroid slows the metabolic rate and causes weight gain, usually a few kilograms plus fluid, while an overactive thyroid causes weight loss, often with a fast heart rate, sweating, tremor and anxiety. In a weight clinic the tests are interpreted carefully, because obesity itself often produces a mildly raised thyroid-stimulating hormone that is not true hypothyroidism. Fat tissue raises the signal that drives the hormone, and when weight falls the value usually settles by itself. Studies have found a raised value in a substantial proportion of people with severe obesity, and that after weight loss or bariatric surgery it falls again along with the need for any thyroid medicine. Between roughly one in eight and one in four people with severe obesity have genuine hypothyroidism, so the test still matters, but a modestly raised result on its own does not automatically mean that tablets are needed. It is also worth knowing what these tests cannot do. A normal thyroid result does not explain or excuse excess weight, and thyroid hormone is not a slimming medicine: taking it when the gland is working normally causes an overactive state with palpitations, muscle wasting and loss of bone strength without producing useful weight loss. If tablets are prescribed for a genuine deficiency, the dose is often estimated from body weight, so the level needs rechecking after large weight loss or after bariatric surgery, and a dose that was correct before the weight loss may need to be reduced.
Medical
Clinical Definition
Thyroid function testing in the assessment of weight change begins with serum thyroid-stimulating hormone (TSH), usually with free thyroxine (free T4) and, where indicated, free triiodothyronine (free T3) and thyroid peroxidase antibodies. Overt hypothyroidism raises TSH with a low free T4 and produces weight gain that is largely fluid and reduced metabolic rate; hyperthyroidism suppresses TSH with a raised free T4 or free T3 and produces weight loss with tachycardia, tremor, heat intolerance and anxiety. Obesity itself is associated with a mildly raised TSH that is thought to represent compensatory activation of the hypothalamic-pituitary-thyroid axis, possibly mediated by leptin, rather than true thyroid failure; indices of central thyroid hormone resistance are elevated in obesity and fall with weight loss. The prevalence of hypothyroidism in severe obesity is reported at approximately 12% to 25%, and obesity is associated with roughly a 70% higher risk of subclinical hypothyroidism, so thyroid function studies are recommended in the evaluation of the aetiology of obesity. A marginally raised TSH should be interpreted only after measuring free thyroid hormones and thyroid autoantibodies and after considering whether clinical features of hypothyroidism are present. After bariatric surgery TSH falls and the total daily levothyroxine requirement usually decreases, while the dose per kilogram of body weight may increase after a Roux-en-Y gastric bypass because of malabsorption, so monitoring and dose adjustment are required rather than assuming stability. Levothyroxine replacement is conventionally started at about 1.6 micrograms per kilogram per day after total thyroidectomy, but dosing on actual body weight in obesity risks over-replacement, which causes atrial fibrillation and accelerated bone loss. Thyroid hormone is not indicated for weight loss in euthyroid obesity, and such use is a recognised form of medication misuse.
Key Insight
Why It Matters
Checking the thyroid is a sensible first step in anyone with unexplained weight change, fatigue or cold intolerance, because hypothyroidism is treatable and its weight gain is at least partly reversible. The important clinical messages are that a mildly raised thyroid-stimulating hormone in obesity is common and often normalises with weight loss, that a normal result does not explain obesity, and that thyroid tablets are not a weight-loss treatment. People already on thyroid replacement need the dose reviewed after substantial weight loss and after bariatric surgery.
Sources
Reviewed against standard medical references.
Medically reviewed by
Family Physician & Registered Acupuncturist, Accord Medical Clinic. MBBS (NUS), Graduate Diploma in Family Medicine (NUS), Graduate Diploma in Acupuncture (Singapore College of TCM).
Last reviewed: October 2026
This page is general health education, not medical advice. Whether any treatment is appropriate for you is a decision made in consultation after assessment.
