Training With an Underactive Thyroid: A Practical Guide to Hypothyroidism and Exercise

How to adjust strength work, cardio and food when your thyroid runs slow: exact sets, reps, protein targets and the signs that mean it is time to see a doctor.

Hypothyroidism does not mean you should stop training. It means your margin for error is smaller: a slower metabolism, slower recovery and stiff muscles mean a program that suits someone else can bury you in three weeks. Here is how to set volume, intensity and food so you actually progress instead of running on empty.

What an underactive thyroid does to your training

Thyroid hormones set the pace for almost every cell in your body. When they run low, this is what shows up in the gym:

Resting heart rate drops, often by 5 to 15 beats. That makes standard heart-rate zone formulas unreliable until your treatment is dialled in.
Recovery takes longer. Instead of the usual 24 to 48 hours, a hard session may need 48 to 72.
Stiffness and cramps, especially in calves and forearms. You need a 10 to 15 minute warm-up, not a 3 minute one.
Fluid retention of 2 to 5 kg that is not fat. The scale lies during the first weeks of treatment.
Cold intolerance. A cold gym means a longer build-up and one more layer.

Medication, food and timing

If you are on levothyroxine, timing is half the job and it costs nothing:

Take it on an empty stomach, 30 to 60 minutes before your first meal, or 3 to 4 hours after your last meal at night. Same time every day is what matters most.
Leave a 4 hour gap from calcium, iron, magnesium, antacids and soy products. These are the most common reason a dose suddenly stops working.
Coffee blunts absorption. Wait at least 30 to 60 minutes.
Never adjust your own dose because of training or dieting. Bloodwork is usually rechecked 6 to 8 weeks after a dose change, then every 6 to 12 months.
Do not start iodine on your own. With some forms of thyroid disease it makes things worse.

Strength: a two to three day framework

Strength work matters more here than cardio, because a slow thyroid hits muscle mass and strength hardest.

2 to 3 sessions per week, full body, with at least 48 hours between them.
4 to 5 exercises per session, 3 sets of 6 to 10 reps each.
Stop at RPE 7 to 8, leaving 2 to 3 reps in reserve. Training to failure buys you nothing here except three days of soreness.
Rest 2 to 3 minutes on the big lifts, 60 to 90 seconds on isolation work.
Total 10 to 14 sets per major muscle group per week. Start at 8 to 10 for the first four weeks.
Progression: 2.5 kg per week on leg exercises, 1 to 2.5 kg every second week on upper body. When you stall for two weeks, add a rep instead of a plate.
Deload every 6 to 8 weeks: same exercises, same loads, 50 to 60% of the sets.

A sample A/B split

Day A: squat 3x6, flat bench press 3x8, bent-over row 3x10, plank 3x30 to 45 seconds.
Day B: Romanian deadlift 3x8, overhead press 3x8, lat pulldown 3x10, lunge 2x10 per leg.

Alternate A and B. On three sessions a week that means A, B, A one week and B, A, B the next.

Cardio: how much and how hard

Aim for 120 to 150 minutes per week of low intensity work, split into 3 or 4 sessions of 30 to 40 minutes. Brisk walking, stationary bike, elliptical. Instead of a heart rate monitor, use the talk test: you should be able to say a full sentence without gasping.

Leave intervals for later. Once your bloodwork is in range and you have gone a month without an energy crash, add one session per week: 6 to 8 intervals of 30 seconds, with 90 seconds of walking between them. Two of those a week is too much for most people with hypothyroidism.

Daily steps are underrated: 7,000 to 9,000 a day does more for your total burn than an extra session you cannot recover from.

Food and body weight

Protein 1.6 to 2.2 g per kg of body weight. At 70 kg that is 112 to 154 g a day, or roughly 30 to 40 g across four meals.
If you are losing weight, keep the deficit at 300 to 500 kcal. A bigger one drains your energy further and speeds up nothing.
A realistic rate is 0.3 to 0.6 kg per week, and the first couple of weeks will look chaotic because of water.
25 to 35 g of fibre and 2 to 3 litres of water daily. Constipation is a common side problem and it wrecks training days.
Selenium comes from food: one or two Brazil nuts a day covers the daily requirement. Do not supplement blindly.

Recovery and the signs you overdid it

Seven to nine hours of sleep is part of the program, not a nice-to-have. Track two things each morning: resting heart rate and whether you actually want to train. If your resting heart rate is 8 to 10 beats higher three mornings in a row, cut that week's volume by 30 to 40%. If soreness lasts more than 72 hours, the previous session was too hard, not productive.

Two weeks with no progress and steadily falling energy is not laziness. It usually means the treatment needs reviewing, not that the program needs to be harder.

When to see a doctor

Resting heart rate above 100, palpitations, hand tremor or insomnia. These can mean the dose is too high.
Chest pain or breathlessness on exertion that was not there before.
Swelling in the neck, hoarseness or trouble swallowing.
Severe muscle pain with dark urine after training. That one is urgent.
Unexplained weight change of 5 kg or more over 2 to 3 months.
You are pregnant or planning to be. Dose requirements almost always change.

The short version

Strength before cardio: 2 to 3 sessions a week, 3 sets of 6 to 10 reps, RPE 7 to 8.
120 to 150 minutes of low intensity cardio weekly; intervals once a week at most, and only once bloodwork is stable.
Protein 1.6 to 2.2 g/kg, deficit no more than 300 to 500 kcal, 0.3 to 0.6 kg lost per week.
Medication on an empty stomach, same time daily, 4 hours away from calcium and iron.
Resting heart rate up 8 to 10 beats for three straight mornings means deload, not push through.