Feeling weak after sex is usually related to temporary physical exertion, dehydration, or low energy rather than thyroid disease itself. However, since you have hypothyroidism and are on long-term Thyronorm, it is important to ensure your thyroid levels are still well controlled. Get your TSH test checked and review your dose with your doctor. Stay well hydrated, eat a balanced diet, and avoid overexertion. If the weakness continues or worsens, seek medical advice for a proper evaluation.